Billy's pet hamster, Mr. Snuggles, celebrates his 10th birthday, so the Grim Reaper comes for him. However, Mandy, Billy's friend, is reluctant in allowing it, so she proposes a deal to the Grim Reaper: if he beats the kids in a game, he can take the hamster, but if he loses, he will spare the hamster's life and become their "best friend forever". The Grim Reaper transports the kids to limbo, where he challenges them to a game of limbo. Despite his best effort, Grim ends up losing because he is attacked by Mr. Snuggles at Mandy’s command.

Billy's mother, Gladys, first notices Grim in Billy's bed, then in the shower and subsequently starts to get panic surges thinking that Billy is in danger. Meanwhile, Harold, Billy's dad, just thinks that Gladys is imagining things. In the end, Gladys ends up going living with Billy's aunt, Sis to try and calm herself down.

Billy and Mandy tell Grim it is "Opposite Day" and that, when they say something, they really mean the opposite. Due to his difficulty understanding the concept of Opposite Day, Grim has to do Mandy's chores. Finally, Grim thinks he understands when Billy and Mandy say they "want" pie, meaning they "do not want" pie. However, Mandy then says that when she said it was Opposite Day, it really was not.

Grim is bored of being left alone while Billy and Mandy are not with him. Grim decides to fulfill his dream by working as a swimsuit model, but, as he is a corpse, he is not hired. Subsequently, Billy and Mandy give him a makeover to look more human. In the end, Grim gets the job in a TV ad, but he is only in the commercial for a few seconds and is portrayed as an ugly person.

Grim misses his life as the reaper of souls and, due to his bet, he cannot return to the Underworld, much to the hilarity of his Underworld co-workers. Having a sudden epiphany, Grim decides to be good and help people rather than try to take their souls. However, Grim's attempts at being good are faced with resistance from people, who are scared off by his appearance.

After watching a movie, Grim teaches Billy to possess people by jumping into their heads. Billy possesses Mandy and makes her do assorted things like flirting with Irwin, kissing Mandy's mother, going into both men's and women's restrooms and eating earthworms. In the end, Billy leaves Mandy's head, but later that night, Mandy possesses Billy's cat, Milkshakes, to maul him as a form of revenge.

Billy wanders into a junkyard, where a massive hole in the earth opens up and pulls him down in. Later, Mandy and Grim get pulled in as well. They land at the center of the Earth, which is home of Nergal, a devilish creature. The lonely Nergal, who has been watching Billy and Mandy for some time, wants Grim to hand over the kids to him so they can be his friends forever. Grim happily leaves the kids behind as he returns to the surface; however, he later goes back for them because he can't handle the guilt of abandoning his friends. In the end, Grim is defeated by Nergal's electrified tentacles, but Mandy defeats Nergal merely by kicking him in the shin.

Billy, Mandy, Grim and Pud'n are part of the "Person Scouts" (a Boy Scouts-esque organization) and need to sell cookies in order to win a badge. However, as Billy eats all of the cookies, Grim proposes to bake their own cookies using his aunt Kali's secret recipe. The cookies are a success and the troupe wins the contest. In the end, at the Person Scouts awards ceremony, aunt Kali reveals that the secret recipe contains insects, but everyone continues eating the cookies anyway.

While Spring cleaning in the middle of November, Billy takes an old, cursed lamp from Grim's trunk. When he gives it a rub, he releases the curse Grim had warned them about: his own mother. Having three wishes to make, Billy wastes his wish by wishing to know what to wish for. Mandy wishes Billy and Grim would "shut up", so Grim never gets to make his wish. Then, Mandy decides to make the last wish to whoever makes her the happiest. In the end, Mandy wishes everyone in the world would go away, which she is glad about.

After having a nervous breakdown and leaving the house in the first episode, Billy's mother, Gladys, returns. She meets Grim again and, this time, she plans to destroy him once and for all. When Grim goes to the kitchen for a glass of water, Gladys attacks him and shatters his skeleton. In the end, Grim assures the kids that Gladys has won the battle but not the war.

With Irwin not answering the phone, Billy's parents missing and Mandy saying she wants "Billy for dinner", Billy and Grim assume Mandy is a cannibal who ate Billy's parents and friends. Looking for proof, Billy and Grim find tiki figures and arrangements for a celebration in Mandy's backyard. Ultimately, it is revealed that they are just celebrating Billy's birthday. Still, Mandy burps out Irwin's glasses.

It's Halloween, Billy, Mandy and Grim go trick-or-treating. Grim mistakes a woman for Atrocia, the host of the all-night Halloween marathon on TV and goes to talk to her. Upon looking at her face, Grim realizes his mistake; however, the woman mistakes Grim for her son, Gregory, and takes him home Gregory is now Grim, and does not have a stomach. At the same time, Billy mistakes Gregory for Grim and he and Mandy take him with them. In the end, Billy and Mandy take Gregory back to his mother and Grim goes back with them.

Nergal, the fiend from the center of the Earth, returns to the surface, emerging a carnival with him, trying once more to make friends. In the following, Billy tries to help Nergal make friends by being considerate, but Nergal fails to apply correctly his advice. When Nergal is told that Grim was forced to be Billy and Mandy's friend-slave, Nergal then decides to force the world to be his friends by turning them into dark beings similar to him. In the end, Nergal turns Billy, Mandy, Grim and even the viewer (thus breaking the fourth wall) into one before the episode ends.

Billy tells Grim to help him throw a surprise birthday party for Mandy. Then, a reluctant Grim helps Billy prepare a cake while keeping it a secret from Mandy. In the end, a somewhat surprised Mandy reveals that her birthday isn't for the next five months, something that Billy already knew about.

Billy and Irwin have been playing their favorite video game, Beasts and Barbarians, for three days non-stop. In an attempt to stop their video game addiction, Grim transports them all to a real-life version of the game. Seeing as Billy and Irwin are still doing good in their quest, Grim increases the difficulty by manipulating the traps himself. In the end, Billy, Mandy, and Grim return home escaping from a giant cyclops, while Irwin remains trapped.

Billy, Mandy, and Grim go to the museum on a school trip. There, they meet Hoss Delgado, self-appointed hunter of the paranormal, who upon spotting Grim, tries to eliminate him. Subsequently, Grim and Delgado engage in a fight. Finally, Delgado quits hunting Grim when he realizes Billy and Mandy control him. In the end, Delgado continues his own adventures and is seen fighting a werewolf.

While going to the shopping mall with Billy and Mandy, Grim spots the beautiful Eris, the goddess of discord and chaos, and tries to ask her out. However, Eris is not interested in the skinless Grim, but in the muscular Adonis. Then, Eris invites Mandy to join her in creating chaos using the Apple of Discord. Although initially they get along, Eris pulls a prank on Mandy, who subsequently tries to get revenge. In the end, Mandy fools Eris into thinking that Adonis is in the movie theater, and Eris accidentally kisses Grim.

Billy eats too much junk food that makes him feel sick. Grim prepares him a remedy using his granny's recipe, but this medicine makes a parasite to start growing in Billy's back, which Billy names Yap-yap. In the following, Billy and Yap-yap continue eating junk food, so Mandy suggest to use the antidote to the original remedy. Eventually, Yap-yap, now looking like a smaller version of Billy, detaches himself from Billy (which would have technically cured Billy without the need of him drinking the anitdote), and tries to escape from the antidote. In the end, Yap-yap drinks the antidote and is destroyed.

Billy is constantly provoked by a bully named Sperg and runs over to Grim for help.

After taking the Underworld's "Bad Book" from Grim, Billy becomes possessed by an evil entity. Then, Billy steals Grim's scythe, and uses it to open a portal to summon Yog-Sothoth. In consequence, Mandy calls for Hoss Delgado to battle Billy. It is revealed that the portal can only be sealed by throwing the book into it. As Billy eats the book, Mandy throws Billy himself into the portal, restoring everything to normal. Instants later, a portal opens up and Billy comes out and says they didn't want him there.

Mandy and her parents leave the city for a day, which makes Billy feel lonely. Then, Grim uses his supernatural powers to create a fake Mandy with pieces of dolls and toys for Billy to play with. However, the "new" Mandy is too aggressive with Billy, and he starts fearing her. The next day, the real Mandy returns home, and promptly destroys the new Mandy while in the see-saw.

In this musical episode, which pays homage to Little Shop of Horrors, Billy befriends a singing brain-eating meteor (voiced by Voltaire) that falls from the sky. With Billy's help, the creature grows bigger and eventually eats the brains of all people in Endsville (turning them into zombies with glowing green eyes). The last brain he eats is Mandy's, which results in the creature transforming into a brain-eating Mandy. The song featured is BRAINS!, composed by goth artist Voltaire specifically for this episode.

Billy, Mandy, and Grim eat moldy pizza leftovers, and after falling asleep, they have nightmares. Billy dreams of cute bunnies which turn into monsters; Grim dreams of an endless number of bossy Mandys; Mandy dreams of being pulled into a volcano and then commanding demons at the rhythm of music, in a scene that reminisces the Night on Bald Mountain segment of Fantasia. In the end, Mandy reveals that her dream was actually very pleasant, and she eats the last slice of the rotten pizza.

Grim gets Billy and Mandy into sorcery school.

Mindy befriends Grim.

Grim brings monster cards to life.

Grim gets sick.

Zombies crave the brownies Billy makes.

Zombies crave the brownies Billy makes.

In the future, Mandy rules the world.

Billy helps Eris spread chaos.

Mandy and Billy are forced to share a room.

Mandy visits a haunted house.

Irwin turns into a werewolf.

Grim falls for a Goth girl.

Mandy develops a crush.

Nergal attempts to woo Billy's aunt.

Billy hatches a giant spider.

Grim tries to help Billy pass a test.

Grim tries to stop a show from being canceled.

Billy and Mandy contend with Nergal Jr.

Grim thinks he is a nun.

Grim and the children enter a kart competition.

Billy tries to show off at the Renaissance festival.

Grim sings in a band.

Billy and Mandy visit the Halls of Time.

Billy and Grim switch places for a day.

Mandy eats a magical ball of chicken.

A headless man from Grim's past uses Billy to steal Grim's scythe.

At a father-and-son picnic, Billy re-encounters with his spider "son", Jeff, in a dirty bathroom. Once again, Jeff tries to make Billy love him as a true father does, but Billy remains terrified of all bugs. Mandy tells Jeff he is being too nice, and to get Billy's attention he needs to be meaner. After being coached by Mandy, Jeff finally snaps and ensnares Billy in his spider web, forcing Billy to say he loves him.

When Billy crashes his bicycle, he obtains a free tricycle from a mysterious boy in a supernatural porta-potty. While Billy loves his tricycle, which he calls Trykie, he is also mocked because of it by Irwin, Pud'n, and Sperg. Trykie moves on its own and attacks the three of them. Mandy realizes the problem with Trykie, and Grim confirms that he is cursed. Billy realizes that Trykie is indeed evil as well, and he lets Mandy destroy it once and for all.

After spilling salt in a restaurant, and not throwing salt over his left shoulder, Billy starts having an unusual amount of extremely bad luck. The next day, Grim analyzes Billy and finds out he has a "Catastrophe Snail" attached to his brain, which is causing him the bad luck.

Billy, Mandy and Grim go to the beach where Grim falls asleep and Billy buries him in sand. When they leave, Billy only takes Grim's skull, leaving Grim's body to be found by a rich family that take it with them. With his adoptive family, Grim's body becomes a celebrity and refuses to go back with Billy and Mandy. Eventually, Grim's skeleton is reunited with his skull, but instants later it is shattered by dogs.

After he turns down his dad's invitation to go fishing, Billy gets a tiny, weakling, pet fish, which he names Little Porkchop. To make Billy happier, Grim makes the fish grow overnight to become a giant. Billy takes Porkchop to play under the Sun, and never gives him water again. He burns to ashes and is flushed down the toilet. Billy's dad returns form his fishing trip having caught Porkchop, now back to his normal size, but plans to cook him.

General Skarr buys a home in Endsville and becomes Billy's neighbor. After Skarr invites Billy into his new house, Billy mentions Grim's scythe as ,he strongest and most powerful weapon in the entire universe, and Skarr tries to battle his desire to be evil again. Eventually Skarr steals the scythe but Mandy proves to be even scarier than he is, and takes it back.

Lord Pain, a fearsome warrior from the Plain of Eternal Suffering, appears in Billy's house looking to serve his master, Grim. Lord Pain is put to clean the bathroom, and is confused as to why Billy and Mandy treat Grim the way they do. Eventually, Lord Pain realizes that Mandy is to be his real master, and prepares mashed potatoes for her.

In this short, set in the Stone Age, a child version of Grim undertakes his first assignments as the Grim Reaper. When he goes to visit a cave witch, she shows him his terrible future being abused by Billy and Mandy

When Mindy mocks Mandy's dumb and coward dog, Saliva, Mandy decides to train her dog to be meaner. After Grim keeps mentioning his own dog, Cerberus, Mandy decides to use Grim's special whistle to call the gigantic, three-headed, fire-breathing hound. Cerberus marries Mindy's chihuahua, Little Delicious, and Saliva snaps against Mindy for a brief moment.

After Billy eats 19 creme-filled Atomic Frenzy Cakes he cannot sleep and wants Mandy to read him a bedtime story. Mandy, Billy and Grim each tell a story. For his story, Grim uses a supernatural storybook from the Underworld, which makes Billy and Mandy enter the book and reprise the tale of Hansel and Gretel. Billy departs from the story and follows Pinocchio, who wants to devour Billy to become a real boy. Mandy saves Billy from Pinocchio, but she and Billy remain trapped in the book as Grim, the storyteller, falls asleep.

When Billy ruins his eyes by playing too many video games and looking directly into the Sun, Mandy and Grim suggest he see an ophthalmologist. When Billy refuses wearing glasses or having laser surgery, Grim uses his own scythe to zap his eyes. Although Billy has sharp and clean eyesight, he can also see glimpses of the future, specially horrific accidents that occur to his best friends. Grim zaps Billy's eyes multiple times to restore them to normal. Billy ends up with weird-looking eyes.

When Billy, Mandy and Grim go playing golf, they encounter boy wizard Nigel Planter again, who is there hiding from the dark wizard Lord Moldybutt. They head back to the School of Sorcery, where they keep on running into Moldybutt, who warns them to stay away from the "Chamber Pot of Secrets" hidden in Dean Toadblatt's office. It is revealed that Lord Moldybutt is actually Toadblatt, who was disguising himself to scare off Planter. Note: The title is a spoof of Harry Potter and the Chamber of Secrets.

When Billy and Mandy are utterly bored, Grim takes them to Underworld's "Circus of Fear". The ringleader, Dr. Fear, notices Billy and Mandy, and tries to recruit them into the circus to attract customers, who would pay to see real, live humans. After they go back home, Dr. Fear captures the two kids, and Grim has to get them back. During the show, Mandy breaks free and in a strange twist of the story, she reveals that the beastmaster, Clortho, is actually Dr. Fear, who in turn is just an earthworm, leaving Billy confused.

After failing to scare a fifteen-year-old, the Boogeyman decides to visit his old middle-school victim, Grim. Grim remembers how Boogie used to be a bully, and demands an apology from him. Boogie disappears to the Underworld, taking Billy with him. Mandy and Grim follow Boogie, and to settle their differences Mandy suggests having a scaring contest, in which whoever can scare a kid the most wins. Boogie's transformation as a werewolf is not enough to scare the kid, but Grim's transformation into a giant, horrific rubber duck is.

As Billy walks into the woods to have a picnic, Yogi and a giant, feral version of Boo-Boo (from The Yogi Bear Show) come to steal his basket, and he escapes by hiding in a cave. Billy is taken to the mushroom lair of the dwarfs where King Beardbottom tells him the story on how the different races split the food-services industry, after the conference of Rivendell. The king sends Billy to infiltrate the elves' cookie factory to deactivate their force field, so the dwarves can assault the factory. After a fierce battle (censored out "for younger viewers", by a single clip of a pink koala continuously licking a eucalyptus leaf) and Billy's words, the elves and dwarves decide to work together and split the profits.

While going to a science museum, Billy and Mandy's school bus breaks down in the middle of the desert. After digging for fossils, Billy, Mandy and Grim wander around and find an abandoned mine, where Billy is captured by a giant, mutant chicken. When Mandy and Grim go to rescue him, they find an underground laboratory from the days of the Cold War where mutant chickens were created. The mountain makes eruption, the chickens are roasted by lava, and the whole class obtains food so they do not have to resort to cannibalism.

Billy's first cousin from the center of the Earth, Nergal Jr. goes to school for the first time, but he is worried that people may not like him. Soon enough, Junior becomes bullied by Sperg, with Miss Butterbean's allowance. Billy suggests Junior to use his shapeshifting abilities to take on the form of Miss Butterbean, so Junior can help him pass a math test. Junior likes the idea, but he morphs into Miss Butterbean to get revenge on Sperg instead. Sperg is seen living in the center of the Earth as Junior's pet.

After watching a TV show, starring Captain Heifer and his Heifer Farms Brand Yogurt, Billy asks Grim to turn him into a superhero with the power of squeaking and throwing yogurt from his armpits. With his wish granted, Billy goes to outer space to fight the Evil Empire, but the planet he visits is mostly inhabited. Despite being free of Billy, Grim still has to deal with Billy's equally dumb father, Harold. After Billy fills the alien planet with yogurt, he returns home to be "normal".

Trying to get revenge at Mandy for bossing them all the time, Billy and Grim put her in the "Mask of the Beast". Mandy must be nice and friendly while wearing the mask, or else she will become a hideous monster. Failing to remove the mask herself, and struggling to be kind to Billy and Grim, Mandy gradually turns into the beast. Billy and Grim ask Mandy to smile while she gives them a sponge bath, but this turns out to be too hard for Mandy. She fully transforms into the beast and punishes them instead.

When Billy sees his father shaving, he decides having a beard is cool and buys "Hair Raiser Tonique", a hair-growing formula. Grim casts a spell on the tonic to make its effects appear faster, but Billy exaggerates the quantity and grows hair in all his body. Billy is mistaken for the Sasquatch, and a reward is offered for his capture. Hoss Delgado also receives the tonic and is captured and exhibited as the Sasquatch, while Mandy collects the reward.

After being bullied again by Sperg, Billy asks Grim to shrink him to enter Mandy's head to steal her "nerve". After Billy swallows the raw nerve, a polyp-like organism, Mandy becomes timid and scared of everything while Billy becomes tough and mean. While Billy goes to get revenge at Sperg, Mandy enters Billy's head to recover her nerve. Mandy recovers her tough and cynical attitude without her nerve, which is successfully retrieved from Billy and banished from Endsville.

When Billy forgets to do his history homework, Grim gives him a universal remote control to alter time and space so he can go back in time and finish his report. Billy and Mandy travel to the American Revolution, to prehistoric times, and even to the future, where they meet an aged Billy who is just finishing the report. After a struggle for the remote, Billy ends up again in the Prehistory and the remote breaks. Fast forward into the present, every person is dumb like he is and shares his characteristic big nose, including Mandy and Irwin.

During a thunderstorm, a lightning bolt hits Grim's scythe, transporting Billy to Asgard, land of the Norse gods. Mandy and Grim go searching for him, while a Viking wearing Billy's shirt remains at his house as a substitute. In Asgard, Billy learns about the endless battle in Valhalla, in which if a warrior dies, he is resurrected to continue fighting. Billy is surprisingly good at killing other Vikings, so Mandy and Grim are unable to convince him to return home.

Grim is annoyed by Billy who thinks that a shadow is following him. Grim takes Billy and Mandy to the Shadows' World, where their opposites live. Grim tells them to do whatever they want, but to be back at 5 o'clock, when the portal to the normal world opens. Grim also warns Billy about the Door to Other Dimensions, but Billy opens it and a multitude of Billys come out an wreak havoc.

After Billy wakes up from another nightmare involving clowns, he is more terrified than ever towards them, so Mandy and Grim try to cure him of this clown-a-phobia. Mandy asks Grim to bring to life a clown's head made on paper mache to entertain Billy, but the spell does not seem to work. They dress up as clowns in an attempt to look harmless. After consulting with his imaginary "Inner Frat Boy", Billy changes his feeling from being afraid to being angry at clowns, and attacks them with Grim's scythe. Billy is cured of his fear, but the paper mache clown attacks and eats them.

Thinking that the world is chaotic enough as it is, Eris, the goddess of chaos, decides to quit chaos to work at a hamburger restaurant. She entrusts Grim the Apple of Discord to watch over for her, which he seals in his old trunk. The apple frees itself from the chest and goes to Billy. Mandy is attracted by the apple's powers, and after being persuaded by Billy's talking burrito, Billy, Mandy and Grim battle for control of the apple, which divides in three pieces. The battle results in them being fused together into an amorphous mass, and, with an evil grin on her face, Eris decides to offer the apple to Irwin too.

When Billy and Mandy have a brief fight, Billy says he never wants to speak to Mandy again. The next day, Billy is over his grudge, but Mandy has already decided to find a replacement for him. Mandy introduces Bobby, the new "Billy", who turns out to be a better friend than Billy, even gaining the affection of his parents, but who also wants to take over Billy's life. A jealous Billy uses Grim's scythe to open a portal and send Bobby to another dimension.

When a doctor is unable to cure Sperg of his severe case of acne, Sperg goes to see a fortune teller that deals with skin problems. Following a Burmese recipe in her book, the psychic tells Sperg to get the Grim Reaper's bones and rub them upon his face. Sperg has to tolerate being friends with Billy and Grim to get access to the latter's bones. One of Grim's arms gets into a stew and when Sperg is immersed in it, the mix cures completely his acne, but it also erases his nose and mouth.

While reading an Underworld comic book, Billy finds an advertisement for "Chocolate Sailors", a delicious chocolate candy-which being Underworld candies are secretly hexed into enslaving the eater. Billy becomes a seller of Chocolate Sailors, but instead of selling them ends up eating them all by himself. The enchanted candies turn him into solid chocolate, and now Billy cannot stop eating himself. Billy, Mandy, and Grim go searching for the Chocolate Sailor to retrieve the antidote before Billy eats himself completely. Instead of eating the antidote, Billy eats the antidote along with other enchanted chocolates, and his head (which was all that was left of him) explodes as a result and covers Chocolate Sailor, Grim and Mandy in chocolate. Later Grim and Mandy are shown watching TV while drinking "Billy" Chocolate shakes.

When Billy has a loose tooth he cannot wait for the Tooth Fairy to appear to give him money. Mandy, who does not believe in the Tooth Fairy, asks Sperg to end Billy's waiting by knocking his tooth off. Sperg's punch leaves Billy unconscious, who has a dream about being in the wild west, where the Tooth Fairy has taken his tooth. Mandy challenges the Tooth Fairy to a showdown to recover the tooth, and she comes out victorious just as Billy awakens. Billy does find money under his pillow, while the aged Tooth Fairy watches through the window.

When Billy hangs out at Irwin's house, he finds a living mummy in the bathroom. Seeing as Irwin gets nervous, and as Billy has never met Irwin's mother, he suspects she is the mummy. To scare him off, Irwin's dad tells Billy he has the "curse of the mummy". Billy is attacked by insects and snakes while he tries to find out the truth about the mummy. It is later confirmed that Irwin's mother is in fact the mummy, but no further explanation is given as to how Irwin's family works or how he was conceived.

Thanks to Mandy, Billy decides to get rid of his "Dinobonoid" action figures thinking he is too old for them. Grim makes the toys come to life, and shrinks Billy to their size so he can have a last adventure with them. The "Jurassic Creeps" become aware of Grim's scythe and steal it from him to use its powers to rule the world; however, the magic that made them come alive runs out just in time, and the toys become inanimate once again. Billy reverts to his normal size too, but he stays trapped inside the box in which the action figures came.

At school, Billy, Mandy, and Irwin look for a club to join. Irwin plans to join the Macrame club to meet girls; Billy inquires on the Junior CIA club, which forcefully recruits him; Mandy joins the "Secret Snake Club", a snake-themed club consisting of three geeks. The nerds reveal that their "secret agenda" is to resurrect Shnissugah, a giant snake which lived hundreds of years ago in Canada, to take revenge against the popular kids, but they lack the power of the Underworld to do so. Mandy calls for Grim, who proceeds to revive Shnissugah through the internet. Billy, as a junior agent of the CIA, stops a criminal plan by Irwin, who was using the Macrame club for personal gain; and Shnissugah turns out to be a tiny, 8-cm (3.14-in) long, harmless snake.

When Billy and Mandy's school is preparing for a big game, Mindy entrusts the care of the school's mascot, Fluffy Cat, to Billy, to protect him from being kidnapped by the rival team, the Booty Heads. At Billy's house, Fluffy Cat is a disaster, so Billy's mom throws him into the basement, where the cat gets inside Grim's trunk, absorbing its powers. A giant Fluffy Cat appears just before the game wreaking havoc.

As Billy is tired of all the rules in his house, he cannot resist the urge to try his father's new motorcycle. The motorcycle breaks down, and Billy asks Grim to repair it. Grim's scythe turns the motorcycle into a wild, supernatural vehicle, which transforms Billy's parents and everyone around it into undead creatures and motorized monsters. Mandy makes Billy tell the truth about breaking the motorcycle in the first place, at which point his parents and everyone else return to normal.

After Billy's baseball team loses badly against Mindy's, Grim decides to coach them. Mandy is not allowed on the team, as Billy is reluctant to let girls play, although he does not realize Mindy is a girl too. Mandy puts on a baseball cap and, assuming the role of "Manfred", pitches for Billy's team anyway. With additional help from ghoulish players which played in the Underworld Little Leagues alongside Grim, Billy's team wins when Pud'n hits a home run.

While at Sassy Cat Land, the theme park dedicated to Sassy Cat (and to ridiculously long lines), Billy, Mandy, and Grim stumble upon a closed attraction, a futuristic "house of tomorrow". Inside, they find Master-Control, a sentient and autonomous, thinking, feeling artificial intelligence computer program claiming to know everything (and wishing death upon the trio when Billy angers him), and Tom Smith, a tour guide that became trapped in the house when it closed forty years ago. Tom explains that the attraction closed when a kid, actually Billy's dad, annoyed Master-Control with his dumb questions, and the robots started attacking the humans. Billy, Mandy and Grim exit the house unharmed, but Tom misses his opportunity to escape and is left behind again.

After a trip to the mall, Mandy becomes the spokesperson for Happy Huggy Stuffy Bears, a new stuffed toy in town. After the success of her commercial, Mandy gives away stuffed bears to other children, who start wreaking havoc in the city. Eris, the goddess of chaos, is behind the toys, which mind-control the children into creating chaos. Mandy decides not to share control of the world with Eris, and orders Grim to destroy the toys.

When Billy finally gets a "secret decoder ring" in his Blasto Bits cereal, he receives a holographic message from a strange creature who speaks about finding the "Secret of the Universe" in an abandoned mine shaft. All the other children of Endsville, also with decoder rings, race to the mine to be the first one to get the Secret. The entire effort is revealed to be a trap laid by Lubbermouth, a worm-like creature from Grim's past, who wishes to free herself from the curse of guarding the Secret. When Mindy looks down at the pit, she becomes the new guardian of the Secret and has to stay there forever.

One night a strange odor travels through the city, prompting the noses of all the people, including Billy's, to follow it. The next night, Mandy, who is noseless, and Grim decide to wait outside Billy's house for the event to repeat. When the odor reappears, the nose parade leads them to the shopping mall, where they meet a "Nasalmancer", a nose wizard who is trying to take over other people's noses since he lost his own. Billy recovers his nose, but slices it in two, and gives one half to the Nasalmancer.

When Billy goes into the mud for the fifth time in the day while wearing his white tuxedo before a wedding, Grim tries to figure out why Billy is so stupid. Billy's dad recounts Billy's life from his birth until he met Mandy and explains that Billy is not dumb, but actually a genius with a certified "Honorary Genius Degree". It is quite evident to Grim that Billy's problem is his equally dumb father.

After Grim's robe comes out of the washing machine, Billy and Mandy find Thromnambular, a talking skull with ruby eyes and a number nine imprinted on his forehead. Grim tells them that the skull is a magical artifact imprisoned on Earth that must grant his nine wishes to be free. When Mandy asks Grim why he has not used any of the wishes, Grim tells her that the skull will corrupt the wishes and end up affecting the wisher.

When Billy gets the opportunity to adopt one of four dogs, he says he wants them all. Grim uses his scythe to magically combine the dogs and create Billy's dream dog, who calls himself Wiggy Jiggy Jed. Although initially Billy and Jed are best friends, their relationship falls apart when Billy tells Jed to sleep outside. Jed uses the powers of Grim's trunk to build a giant machine to capture the beds of all people. Billy throws Jed a dish of primordial ooze and he dissolves.

To raise money to buy Sea-Critters (similar to Sea-Monkeys), Billy has a garage sale where he sells Grim's scythe to Irwin for less than two cents. With the great mystical strength and power of the scythe, Irwin hypnotizes Mandy and takes her on a romantic date. Billy and Grim try to recover the scythe, and when they do Mandy gets out of the trance. Irwin is punished by being transformed into a tiny sea-critter and placed in Billy's fish bowl.

When Billy gets under his house's foundation, he encounters again his "son", Jeff the Spider, who has just laid eggs. Jeff tries to make things right with Billy so he can take care of the baby spiders, as he is going to die after the eggs hatch. Naturally terrified of seeing Jeff, Billy runs away and a wild car chase ensues. The eggs hatch, and Grim decides to refill Jeff's hourglass that represents his lifetime so he does not die and can continue to scare Billy.

After seeing Billy, Mandy and Grim battling a strange creature, Irwin again feels rejected by Mandy. Hoss Delgado, spectral exterminator, appears to help Mandy destroy the monster. Delgado decides to have Irwin as his sidekick and train him to be "cool" to win Mandy's love. Irwin gets an attire like Delgado's, and attaches an egg beater to his right hand. Delgado releases a monster so Irwin can fight it and impress Mandy, but he is humiliated anyway. Mandy tells Irwin he is just a nerd, and Billy tries to assure him that being a nerd is not such a bad thing after all.

After Grim has a nice dream, a nightmare escapes the back of his head in the shape of a supernatural, invisible duck that makes farting noises. The duck follows Grim everywhere, causing him embarrassment with the non-existent farts, and to end up in jail. The duck repeats the strategy, following other citizens, including Irwin and Billy's dad, resulting in all of them going to jail too. The duck tries to annoy Mandy, but she remains imperturbable by its presence. The duck disappears and Principal Goodvibes is really angry, and he gives Mandy 75 days of detention.

After being utterly bored of watching the same movie 60 times in a row, Grim takes Billy and Mandy to "Underworld Video" where they rent "The Legend of Chupacabra". While Billy watches the tape, a small, pink chupacabra emerges from his television and Billy decides to keep it as a pet, naming it Daisy. Mandy and Grim convince Billy that he is inept in taking good care of pets, and they return the blood-sucking chupacabra to the videotape.

Billy and Mandy find a supernatural zipper from Grim's robe, and while Billy uses it to create mischief, Mandy gets herself in a sticky situation.

When Billy jumps into a supernatural puddle (a Cosmic Sinkhole, according to Grim), it takes him to the home of Granny Applecheeks, who wants to bake him into a pie. It is up to Mandy and Grim to save him if the puddle can take them to the right place.

At school it is time for the "Presidents Fitness Test", but Billy is absolutely unfit for exercise. On the other hand, Nergal Junior uses his shape-shifting powers to become fit at will and excel in every test. Billy asks Junior to transform into a pair of pants that he can wear, so he can run fast and win the 50-meter race. Since Junior has never attempted to transform into inanimate objects he cannot stop running and cannot revert to his human form. Billy keeps running forever while he becomes a national sensation known as the "Fitness Lad". It is revealed that the story was just all a fabrication created by the narrator, who parodies The Twilight Zone.

After Grim's scythe fails to open a portal, he decides to sharpen it, but the blade breaks. Billy, Mandy and Grim go to the Underworld to meet F (a parody of James Bond's "Q"), who is in charge of the Underworld secret gadgets. F unveils Scythe 2.0, which is a new-and-improved sentient, autonomous, thinking and feeling computerized scythe, that Grim uses while his classic scythe is being repaired. When Mandy tells Grim to do his usual humiliating chores, Grim delegates the work to Scythe 2.0, which rebels and decides to reap by itself, starting with Billy and Mandy. Grim saves the kids, takes Scythe 2.0 back to the Underworld, and returns with his old scythe.

When Billy buys a new cereal, a phoenix emerges from the box and attaches to his head, annoying everybody. Since the phoenix cannot be removed because it revives every time it is destroyed, Billy, Mandy and Grim seek help by going to the factory where the cereal is produced, which is run by Eris, the goddess of chaos. Mandy tricks the phoenix into leaving Billy and attacking Eris instead. It is revealed that all the events were really an episode-within-an-episode.

When Billy desperately wants chewing gum at school, Grim gives him a bottomless gumball machine, but warns him not to swallow the gum. Billy ignores Grim's warning and starts swallowing the gum, causing him to constantly burp bubbles that fill the entire city. Mandy and Grim try to get Billy to cough the remaining gum that he has in his stomach, but instead Billy swallows the entire gumball machine. Billy coughs huge amounts of gum all over the city and ends up with a supernatural rash from another interdimensional world.

When Billy is accepted to Mrs. Pollywinkle's dancing academy, his father tries to persuade him to forget about dancing because of his own bad experience as a child during a recital. At the academy it is revealed that Mrs. Pollywinkle is actually a powerful, dark witch set on stealing Billy's soul. Billy's dad unexpectedly comes to rescue, challenging Mrs. Pollywinkle to a dance competition, which he wins using a powerful dance move. Billy's dad tells Billy to follow his dream of becoming a dancer if he so wishes, but Billy changes his mind saying that dancing is for girls. Note: This episode was inspired by the movie Billy Elliot.

When Grim misuses his powers to play bowling with asteroids, he is taken to the Underworld court and sentenced to 50 hours of community service. At a retirement home, Billy, Mandy, and Grim go to the supernatural wing, which is the Home of the Ancients, a place for retired, geriatric monsters. Grim meets his childhood idols, the Bride of Frankenstein, Dracula, and the Wolf Man. Despite Billy's comments that the old monsters are finished, Grim sets out to prove that they are still scary. Grim tries to scare a bunch of kids in the woods, but the old monsters ruin his attempt and he walks out on them.

When it's scurvy-prevention day at school, Mindy parades in her float in anticipation for the Little Miss Scurvy beauty pageant, and she insults Mandy, telling her she is ugly. To prove her otherwise, and with Principal Goodvibes' encouragement, Mandy considers entering the pageant. Naturally, Billy and Grim cannot believe Mandy when she tells them about the idea, but finally they decide to help her. Grim calls for Crabina, the Underworld's greatest makeup artist, to give Mandy a complete makeover. Crabina is shocked by Mandy's sour attitude, so she sets up principally to make her smile. At the pageant, Mandy has a real chance at winning the contest, but she still refuses to smile during her heart-warming musical number. Ultimately she smiles, but this event is so powerful that it alters the laws of the universe, and The Grim Adventures universe transforms into the The Powerpuff Girls universe, with Billy, Mandy and Grim turning into the Powerpuff Girls, Irwin turning into Mojo Jojo, and Professor Utonium guest starring as himself in a brief speaking cameo.

When Billy's parents are attacked by an anaconda, Billy suggest visiting boy wizard Nigel Planter, because he can talk to snakes. Planter in fact does not speak to snakes, but to snacks, and instead he asks for Billy, Mandy and Grim's help to win the affections of Huffefnie Pfefferpfeffer, a beautiful witch in the School of Sorcery (and having a striking resemblance to Gladys). Planter tries using a romantic love potion and enchanted chocolates, but they do not work. The guys end up completely forgetting about Billy's parents.

When Billy's parents leave for their second honeymoon, Billy is left alone at the worst possible moment, since he has invited Principal Goodvibes over to dinner so he can recommend him into Rollington Academy for the Gifted. Billy's dad suggest Billy to use Eris, the goddess of chaos, and Hoss Delgado, spectral exterminator, as his substitute parents during the dinner. The dinner is a disaster with Delgado terrorizing Goodvibes, and Eris unleashing giant alien zombie lobsters. Goodvibes forcefully recommends Billy into Rollington Academy, which, as it turns out, is actually a military school.

Then Billy's mom, discovers crow's feet on her face, she panics and does not want to leave the bathroom. As Grim needs to use the bathroom, he uses his scythe to make Gladys a young girl again. Gladys goes to see Billy, but neither he nor Mandy nor Irwin recognize her, and instead, Billy calls her "Corn", because of the white bread she gave him. When Corn tells Billy to do his homework, Mandy tells him to go to the movie theater instead. Corn and Mandy struggle over who gets to control Billy. Billy's father plays with Grim's scythe and also becomes a child; he then goes to the movies and gets together with Gladys (Corn) again.

When Billy, Mandy and Grim notice things have been disappearing in the house, they set out to investigate using a supernatural detector. They find the culprit is a talking "taking tree" in Billy's yard, who has been stealing objects since Billy's dad was a child. To make the tree happy, Mandy and Grim set out to trade the stolen belongings for a highly-valued Mickey Munchle baseball card, which is worth eleventy million dollars. Mandy and Grim return from the baseball shop with the real Mickey Munchle. The tree returns all of the stolen things, but keeps the player.

When the mall's surveillance cameras capture Grim's silhouette attacking endangered owls, a naturalist named Corey Nebraska blames Grim for the attacks, while Mandy suspects Grim is sleepwalking again. Billy, Mandy and Nebraska follow Grim while he sleepwalks, but they realize it is actually Billy's mom, the one attacking the owls. When discovered, Gladys reveals her hatred toward the owls as she recounts the story on how an owl ate her pet rat, Sniffles, years ago. Billy's dad reveals that he was in fact the one who ate Sniffles merely to survive until lunch time.

When Nergal goes to visit Billy's family (his in-laws) for the weekend, he ends up staying with them over a year. When Billy finally asks his uncle to leave, Nergal says he cannot return home because he is a complete failure, having lost at a board game 599 times in a row against his own family, and experiencing constant disappointment elsewhere. Billy and Grim tell him to not worry about being a failure, but to embrace it. After training with Billy and Grim, Nergal returns home and he finally wins at the board game, but he also makes a crack in the ceiling and his home starts filling with lava.

As Billy desires to be "cool" like Sperg, he has an ectoplasmic manifestation coming out of his mouth. The ectoplasm grows full size and turns out to be the ghost of nineteenth-century British poet Lord Byron, who tries to teach Billy to be cool, using elaborate poetry instead of bare muscles. Billy uses a poem to insult Sperg's mother, and Sperg challenges Billy to a duel. Billy and Lord Byron fight Sperg and his gang, but are badly beaten. Byron tells Billy he has been trained in the arts of coolness, and disappears.

As Grim watches the entire season of "My Troubled Pony" (parodying My Little Pony), he learns from Mandy that the show was cancelled and does not have an ending. Grim decides to reap the "Schlubs" (a parody of The Smurfs), 100 creatures that live in Billy's yard, to melt them into gold so he can finance the creation of the series finale. Grim must get past Billy and the aggressive Schlubs themselves, including the single, large female (a parody of Smurfette). After Grim thinks he got the creatures with his scythe, Mandy appears to have used all the gold herself to become rich.

When Billy and Irwin start making prank calls, everybody knows that Billy is behind the calls because every telephone has a caller ID. While Billy searches in Grim's trunk in the basement for something to defeat the caller ID, he finds a skull-shaped phone with no caller ID. Grim warns him to not use the phone, because it is used to call the ancient demon Cthulhu, whose sole appearance makes people go mad, and if awakened will cause the destruction of the world. Despite Grim's warning, Billy and Irwin use the phone anyway, and they are sucked into it by tentacles.

The day before Christmas, Billy, Mandy and Grim go to the shopping mall, where Mandy complains that Santa Claus is just an invention from corporations. Grim assures Mandy that Santa is real, and they even went to college together. They decide to travel to the North Pole and meet Santa, but when they arrive Mrs. Claus tells them that Santa Claus has been transformed into a vicious vampire.

When Billy, Mandy and Grim go to the beach, Billy is swallowed by a whale known as Blubbery Joe, which also houses in her stomach Mister Geppetto and Pinocchio. Tired of not having a real son, Geppetto convinces Billy to be his "real son", while Pinocchio becomes increasingly jealous and is set on eating Billy's flesh to become a real boy and gain Geppetto's fatherly love and affection. Mandy and Grim search for Billy on board Captain Deadwood's ship, who is made of wood and who has been hunting Blubbery Joe for years. Billy, Geppetto and Pinocchio are expelled from Blubbery Joe's stomach, and as Billy needs to go home, Blubbery Joe accepts to be Geppetto's son.

When Mandy and Grim tell Billy to quit watching TV and follow his life dreams, Billy gets into a chicken costume and decides to become a world famous "chicken juggler". When Billy's mom tries to convince him to be a chiropractor instead, Billy runs away to Uncle Chokey's farm to pursue his dream. Billy and Uncle Chokey juggle chickens at the rhythm of an old phonograph, whose music turns the farm into a 1920s black and white rubber hose cartoon. Due to a shortage of eggs in the city, Mandy and Grim go to the farm to investigate. They destroy the phonograph and things return to normal.

When Billy's dad gets a promotion at his job, he tells his family they are going to move to another town, starting a dispute between Billy and Mandy over who gets custody of Grim. Grim suggests they settle this in court, and hours later Billy, Mandy and Grim find themselves in the Underworld court, presided by Judge Roy Spleen, and which has Fred Fredburger, an incorrigible dumb character, as a juror.

When Billy decides he wants to grow up to be like Grim, he says nothing will stand in the way of their friendship. They meet a girl named Blandy, who looks very much like Billy, and Billy falls completely head-over-heels in love with her. When Blandy shows romantic interest in Grim, Billy is struck with mad jealousy and wants Grim out of the picture. Mandy consults Grim's encyclopaedia of monsters and realizes Blandy is actually a creature from another dimension. Despite Blandy showing her true form and leaving, Billy remains madly romantically infatuated with her; but only up until the following episode.

Billy, Mandy and Grim visit Mighty Moe, an old man who has discovered the secret to add extra years to his lifespan: a good diet and exercise. Mighty Moe decides to publish his secret in a book, and Grim needs to stop him by offering junk food.

When Billy digs his front yard he finds a frozen caveman, who is actually Fred Flintstone (from The Flintstones), but whom Billy names Jake Steele. Billy tries to acclimate Fred to the modern life but the job is not easy. When Fred is taken to school, he predictably goes out of control, kidnapping Miss Butterbean and running away in Principal Goodvibes' car. The ensuing chase results in Fred crashing into an ice cream truck, becoming frozen again. Instead of reaping him, Billy decides to bury Fred in the yard again. A future is shown where both Billy and Fred are defrosted by brain-eating creatures.

When Billy and Irwin want to watch classic giant monsters movies, Grim decides to take them to Japan to see real monsters. They meet the turtle-like Cragorah (a parody of Gamera) who engages in a fierce battle with the three-headed Kittirah (a cat version of Ghidorah). To help Cragorah, Grim transforms Billy into a giant monster too (a parody of Ultraman). Mandy soon arrives with the intent of punishing them all for organizing this expedition without her permission. Controlling Mecha-Gorillasaur (a giant gorilla robot) she defeats Kittirah and beats Billy.

When Billy and Irwin are yet again bullied by Sperg, they decide to play for the school's football team so they are not bullied anymore. During practice, Irwin shows a powerful arm and he becomes the starting quarterback, while Billy is relegated to being a punching bag. During their first game, Mandy directs the team into scoring their first ever touchdown, so Coach Kilgore gives her the job of captain cheerleader, taking it away from Mindy. With Irwin's arm and Mandy's cheerleading the team reaches the finals. Mindy tries different ways to get back at Mandy and recover her job, but she ends up affecting her own team, which eventually loses the game.

When Billy gets his kite stuck in a tree, he starts throwing objects at it to knock it down, including Mandy and a group of three boy scouts, but all objects get stuck too. Frustrated, Billy and Grim call a druid for help, who in turn trains them to become druids, so the three of them can perform a ritual. Meanwhile, inside the tree, Mandy battles a group of squirrels, and leads the scouts back to the ground. Since the druidic chants did not work against the tree, the druid goes ahead and uses a chainsaw to bring the tree down, but this gets him arrested.

When General Skarr tries to get rid of a weed in his otherwise perfect garden, he asks Grim to use his scythe to do the job. Billy drives a lawnmower and ruins the garden; also, the scythe's magic accidentally shoots Skarr, transforming him into a plant-like creature. Using his new plant powers, Skarr rebuilds his garden, and tries to exact revenge on Billy by throwing a party, and attacking him. Skarr gets mulched by a helicopter rotor, but he is reborn as a small weed.

When Mandy and Grim go to the warehouse store, Mandy notices that everything seems to be in order, lacking any presence of chaos. Hoss Delgado, spectral exterminator, arrives saying that everything has been that way since he started dating Eris, the goddess of chaos. Mandy assures Delgado he must break up with Eris to restore chaos, but fearing her temper, he is reluctant to do so. After failed attempts by Grim, Mandy, Billy, and Irwin to break their relationship, Delgado finally tells Eris the truth about he wanting to break up, and she complies by turning into a giant insect and devouring him.

When Billy goes to visit his maternal grandmother and Mandy decides to have the day for herself, Grim finds himself free of the kids to do whatever he wants to do. Since Grim quickly gets bored he goes to visit General Skarr, and lets him take his magical scythe, which Skarr uses to build an army of giant lawn gnomes. Since Grim is so used to having two kids with him when they go on an adventure, he enlists Mindy and Pud'n to help him combat Skarr and retrieve his scythe. Mindy and Pud'n prove to be worthy substitutes but they are scared away by Mandy when she notices the unusual trio

There is a new student in the school, named Dora (parodying Dora the Explorer), who insists on getting people to open her lunch box. Mandy befriends Dora once she gets into a fight with the snobby Mindy. While noting something strange with Dora, Grim does not recognize her as Pandora until it's too late and she tricks Mandy into opening Pandora's box, releasing all kinds of plagues upon humanity. Mandy restores everything to normal, and captures Pandora inside the box, saying that nobody tricks her, and that when the time is right it will be Mandy herself who will unleash humanity's doom and destruction.

When Billy destroys Grim's favorite childhood toy, Fancy Jake, Grim feels devastated and wishes he had never lost the limbo game that made him become Billy and Mandy's best friend forever and ever. A lawyer appears saying that although the game is legally binding while all parties remain on Earth, it is not if Grim leaves Earth. In consequence, Grim follows the lawyer's advice, opens a mystical portal, and leaves. With Grim traveling aboard his spaceship, the lawyer takes off his disguise and introduces himself as Morg, the Martian Grim Reaper, who is also Emperor of Mars and commands an army of Martian, brain-eating zombies.

As Billy sees Hoss Delgado's manliness is action again, Billy wishes to grow up to be a hero like him. Grim tells Billy that in older times an evident way to become a hero was to slay a dragon. They travel to the Middle Ages so Billy can train under the supervision of the best dragon slayer, Sir Boss Del Guapo, who resembles the present time Delgado. Billy becomes friends with the dragon he was supposed to kill and spares his life, becoming the dragon's hero. Del Guapo finds himself transported to the present, where he has a brief encounter with Delgado.

When Grim decides to visit again his childhood idol, the aged king of the vampires, Dracula, the latter suggest they drive to an early bird buffet, only to get lost in the scorching desert. Grim and Dracula need to find their way back home while dealing with situations like meeting a creepy truck driver and being attacked by a giant scorpion. After what they have gone through, and thinking that Dracula is just a selfish, old man, Grim decides to go a separate way. After finding that Dracula in fact helped him by sucking out the scorpion's venom, Grim rushes to help Dracula, and they finally make it to the diner. However, it appears that Dracula left without paying the check.

As Billy and Mandy celebrate Father's Day, Grim becomes very depressed that he sits inside Billy's closet only listening to country rock music. Grim explains that, since his own father cared more about country rock than his own son, he has been living a lie; telling his father he is a famous country rock singer, and not the actual Grim Reaper. Mandy arranges for Grim's father to come visit and she tells him the truth, but still the old man does not believe her. Grim decides that the only way for his dad to listen to him is while performing country music. At an outdoor concert, Billy, Mandy, and Grim start performing a country rock song, in which eventually Grim reveals his supernatural abilities as the mighty Grim Reaper to his father. Grim's dad accepts his son is the Reaper, but he still does not forgive him for lying to him.

As Mandy returns from Billy's house, it is evident that Mandy's parents are extremely afraid of their daughter and her iron fist rule over them. Unable to live in fear any longer, Mandy's parents call a nanny from a reality TV show who specializes in disciplining spoiled children. Mandy and the nanny clash while the nanny tries to teach Mandy's parents to show authority to control their daughter. Grim helps the nanny in her feud against Mandy by using his scythe to make Mandy's parents more controlling and resolute. Mandy quickly restores her tyrannic control over her parents, and shows the nanny she actually controls everything and everyone.

When Mandy says even a trained monkey is more competent to do cleaning chores than Grim, and would complain less, Grim makes Dickie Galoot, a monkey appearing in a Japanese-styled animation, come to life to prove her wrong. As Dickie turns out to be extremely good at cleaning Mandy's house, Grim decides to leave the kids for good. Due to his friendship contract, Grim is unable to break free, and he is forced to battle Dickie to recover his place as the "cleaning monkey" of the group. Grim successfully defeats Dickie at cleaning Billy's house, but this all turns out to be an ingenious plan devised by Mandy to win a bet against Billy.

At a school fair, Grim's scythe accidentally transforms the dull Principal Goodvibes and his hippopotamus into gangster rappers, Goodvibes now calling himself Principal Goodbling. In the following, Goodbling changes the entire school to be more hip-hop-oriented, to the annoyance of Mandy. Although initially Grim likes the new style of the school principal, he later thinks Goodbling is just a poseur and goes to talk to him, but Goodbling attacks with yo-mama jokes. Irwin's grandmother, Tanya, confronts Goodbling and proves to be better than he is at yo-mama jokes that Goodbling reverts to being the boring Goodvibes.

When Billy finds his "son" Jeff the Spider lying in his bed with "Spider Pox", he goes to Mandy for help to get rid of the giant spider. While ignoring Billy, the next morning Mandy awakens afflicted by Spider Pox too, developing four additional arms and the ability to produce spider web. Mandy's condition progresses and she completely turns into a giant spider, who captures Billy's parents and Grim. Grim tells Billy that, to cure Mandy, he must use the venom of the spider that infected her. While Billy successfully gives Mandy Jeff's venom, he also carried the virus and infected the people of the city, who have all turned into spiders.

As Grim appears in an Underworld TV commercial for "Big Move" laxative pills, an accompanying contest promises a day with the famous Grim Reaper to the monster who writes the best 500-word-or-less essay describing the fun things they would do. It turns out the winner of the contest, and sole participant, is the dimwitted Fred Fredburger, who only wants to eat frozen yogurt. The Big Move company blames Grim for their sales going down since they hired him. In consequence, Grim tries save his reputation and make the best day for Fred, but it is a disaster with Grim ruining Fred's yogurt and Fred going missing in an amusement park. The Big Move executives give Grim a beating for ruining their company, and Fred finds his own paradise in a cold place with nachos- and frozen yogurt-loving creatures.

As Billy notices Mandy and Grim's usual negative attitude, he wishes they were in a magical land of happiness, so Grim transports them to an enchanted forest. Billy enjoys the relaxed surroundings until he meets a cantankerous and sour unicorn named Mary-Frances. Billy tries to change Mary-Frances attitude, but she is reluctant due to an earlier trauma of not being able to swim and follow the other unicorns into the sea. After Billy teaches Mary-Frances to swim, she feels happy and finally can join the other unicorns, but they turn out to be bullies, the real reason she never joined them in the first place. Billy stays with the unicorns and Mary-Frances goes back with Mandy and Grim to watch TV.

As Billy, Mandy, Grim, and Irwin hang from a rope about to be eaten by a giant sea serpent, Irwin wonders how they came into that trouble. Billy and Grim each tell separate stories on how they originally became best friends forever and ever. Billy's fantastic story is about him and Mandy being space patrol officers who collect the Grim Reaper's bones, cloak, and skull to summon him; Grim tells a story on how the evil Mandy and his pet servant Billy managed to capture him inside a containment unit (parodying the Ghostbusters' ghost traps). Mandy quickly tells the real story, and footage from the first episode is seen, before the four are swallowed whole by the serpent.

As Billy behaves more reckless than ever and cannot stop breaking everything in sight, Mandy thinks he should go some place where he can release all his urge to destroy until he gets tired of it. Mandy, Grim, and Irwin go to Level 9, home of the fearsome warrior Lord Pain, to convince him to take Billy. Billy receives psychological treatment and it is revealed that his desire to destroy stems from lack of attention when he was younger. Lord Pain abducts him, and together they start destroying everything in Level 9, which seems to be is uninhabited. Billy is still not satisfied, so he and Lord Pain return to Endsville and carry on breaking everything.

When Grim leaves the house to meet someone he met on the internet, Billy looks inside Grim's trunk for something to entertain himself. Billy finds a magical talking Rubik's Cube, which releases the demon Pinface, who has bowling pins in his head. Pinface proceeds to call his minions to conquer Earth, but the minions are more interested in throwing a house party. It is revealed that Grim and Pinface already know each other, and Grim even broke up with Pinface's sister. Billy's father ends up cheating and solves the magical cube by rearranging the stickers, trapping Pinface and his minions inside it again.

As Billy realizes the three geeks of the Secret Snake Club get doctor's notes to avoid Physical Education (P.E.) class, he wants to know their secret to avoid the class too. All the boys in the class join the club and all, except Billy, get doctor's notes from Billy's dad, who poses as a doctor. While Billy is the only one exercising in the class, the Snake Club summons a snake to destroy the gym. Since the invoked snake is tiny, Sperg uses Grim's scythe to make it a giant. Mandy and the other girls beat the boys and their snake because they did appreciate the P.E. class as an outlet for their aggression.

When Billy, Mandy and Grim attend an Ancient Egypt exhibit at the museum, they meet with Irwin who explains that, as his mother is a mummy, they are visiting Irwin's grandfather, the great Pharaoh King Popen Locken. Later that night, Irwin's grandfather visits Irwin in his room and offers him the Pharaoh's crown and the possibility of building his own pyramid to attract women; Irwin accepts, calling himself King Tooten Pooten, and soon they turn the citizens into slaves to build the pyramid. Against his mother's wishes, Irwin decides to finish the pyramid; however, there is something he did not anticipate, as he, the Pharaoh, needs to remain entombed inside it.

When Billy fails an important test, the result of which will go into his permanent record, he asks Grim to magically alter his grade from an "F" to an "A"; as Grim reluctantly changes the grade, Billy becomes a genius, but reality is thrown completely off balance. Mandy and Grim go back in time to stop the past Grim from helping Billy, but it does not work. They keep going back in time to different epochs, each time meeting another Grim, eventually trying to stop Billy's ancestors from conceiving him, but nothing works. After meeting Billy's distant dinosaur relative, the episode ends abruptly as Grim wonders what the end of this could be.

After watching the Unsolvable Mysteries TV show, Grim realizes he forgot a million years ago to reap the Abominable Snowman living in the Himalayas. Billy, Mandy and Grim go there to finish the job and also to obtain videographic proof of the snowman's existence. The Yeti, who likes to be called Abom, manages to escape Grim more than once before finally being trapped by the moving glacier. Grim watches the TV show again, but he may have forgotten to reap the Loch Ness monster too.

When Nergal comes to tell the guys about his new pizza joint, Grim thinks he can do better by using his Granny's recipe. A pizza war ensues between Nergal's Pizza and Grim's Pizza, with each pizzeria launching TV ads and discrediting each other. Nergal Jr. uses his shape-shifting abilities to infiltrate Grim's pizza and replace Granny Grim's hot sauce with a "ghastly elixir". The result is pizzas becoming monsters and attacking the city, and the citizens developing parasites similar to Nergal. Nergal and his family leave the city just as Grim's sauce explodes.

After Billy, Mandy and Grim are disappointed with the Sassy Cat amusement park submarine attraction, Grim uses his powers to take them to the bottom of the ocean to have a real magical adventure. They are sentenced to death for trespassing into the Googen Kingdom by King Triceps, a wrestling merman. The king's daughter, Hariel, also a mermaid, stops the sacrifice and tries to marry Billy to inherit the kingdom. It is revealed that King Triceps is actually Hariel's sister, the sea witch Uvula, who also tries inherit the kingdom. A cat fight ensues between the two ladies, which ends when Billy agrees to marry both of them. The plot is revealed to be a story Billy is telling to Miss Butterbean, who initially asked him what happened to his report.

As General Skarr is once again annoyed by Billy, Hector Con Carne and Major Dr. Ghastly arrive in his house with the intention of luring him back to the recently relaunched Evil Con Carne world domination organization. Although initially refusing to rejoin, Skarr later accepts with the condition that Billy and the other neighbors be destroyed. After a few weeks of living together and preparations, Evil Con Carne's ultimate weapon is revealed to be an army of tanks that shoot giant rubber bands. Skarr's all-out attack over the neighborhood is foiled when Billy tricks Hector into shooting Skarr's garden and they get into a fight.

As General Skarr has trouble finding a job, the recruiter tells him to attend anger management classes if he wants to work again. Skarr finds out Mandy and Grim are attending too, Mandy for staring at the school monitor, and Grim for enjoying his job too much. The instructor of the class, Larry, turns out to be a peace-loving, short man riding a tricycle, who teaches the trio ways to calm down when angered, including counting to ten, and using harmless weapons to let go of their anger. Larry invites Billy to the class, and Mandy, Grim, and Skarr all manage to contain themselves; however, Larry himself suffers an episode of rage over a parking space and he is arrested.

When Mandy finds Billy playing with Grim's gigantic dog, Cerberus, she tells Grim to keep Billy under tight control because she needs a good night of sleep to be ready for the next day's test. With a little persuasion from his own dad, Billy tries to wake up Mandy at night, so she will fail the test and he can score higher than her for the first time. After Grim spends the night guarding Mandy's house from Billy, the latter succeeds in waking up Mandy. It is revealed that this Mandy is in fact Irwin, used as a decoy by the real Mandy, who slept in her dog's house. Mandy passes the test, and Grim is punished for not guarding her properly.

When Billy, Mandy and Grim once again go to the beach, Grim warns Billy to avoid swimming too far, because the "undertoad", a frog-like creature, will try to drag him into the sea. Billy encounters the undertoad, and he barely makes it back to the beach. With the dangers of the sea averted, the guys go back home, but the undertoad is set on bringing Billy down, that he follows him to his house. The next day, the undertoad becomes a constant annoyance to Billy, even at school. Grim and Mandy capture the undertoad, and set sail to release him at sea, where they meat the giant Mama undertoad.

When Irwin lacks the courage to ask Mandy to go with him to the Valentine's Day school dance, he tells Nergal Jr. to do it for him. Mandy mistakes Junior's invitation for his own, and surprisingly accepts to go with him, which naturally makes Irwin jealous. With the ill advice received from his father, to be possessive with Mandy, at the dance Junior becomes overprotective of her, that eventually he and Irwin start fighting, throwing punch at each other and ruining the ball. It is revealed that this was Mandy's true intention all along so that no one could enjoy Valentine's Day.

When Billy accidentally food-poisons Miss Butterbean, Principal Goodvibes offers Grim the job of substitute teacher, which Grim accepts as now Billy and Mandy have to obey him. Grim sends misbehaving students to the "Detention Dimension", a place of eternal classroom punishment, until only Mandy remains in his class. Grim tricks Mandy into opening a portal and she is dragged into the dimension too. Mandy proves to be more terrifying than Miss Slither, the giant, rattlesnake-like creature in charge of the Detention Dimension, that Mandy escapes the place and takes revenge on Grim by sending him there too.

When Billy is sent to bed after playing with his Dr. Brainiac action figure, he wishes he had an adventure. He notices a strange light outside and wanders into a corn field to investigate; he encounters Sperg, and both of them got caught by a UFO's tractor beam. Inside the extraterrestrials decide to experiment on them.

When vampire hunter Lionel Van Helsing prepares to kill Dracula, Billy, Mandy, and Grim arrive to stop him since Dracula is just senile and harmless. Van Helsing reveals the source of his hatred: in the past, he and Dracula were "best friends forever", until Dracula used his irresistible dance moves to marry Tanya, the woman of his dreams. Tanya arrives too, revealing that she is none other than Irwin's grandmother, making Irwin one-quarter-vampire on his father's side and half-mummy on his mother's side. Van Helsing leaves, but Dracula is unsatisfied with his newfound family of nerds.

As Billy, Mandy, and Irwin need to write a school report on tall tales, Grim tells outrageous tall tales about Pecos Billy (based on Pecos Bill), Paula Bunyan (based on Paul Bunyan), and John Henry. In Grim's stories, Pecos Billy (Billy) tames a twister using his personal laser-shooting cow-like spacecraft, saving an Arizona trailer park; Paula Bunyan (Mandy) uses her blue ox companion, Babe, to stomp over a villain, creating the Grand Canyon in the process; John Henry (Irwin) digs through a tunnel so fast that he breaks the laws of physics and bursts into the "eighth dimension". After Grim tells them the stories, Mandy informs Billy and Irwin that he is trying to get them F's.

When Billy, Mandy and Grim go to a joke shop, they find the owner is former boy wizard Nigel Planter, who reveals that his powers diminished and was kicked out of Toadblatt's School of Sorcery. Planter steals Grim's scythe and heads to the school to exact revenge on those who wronged him. The guys seek the help of Planter's sworn enemy, Lord Moldybutt, to find the school. Grim recovers his scythe, and Moldybutt turns out to be in fact only a real estate agent, who informs Planter that he is the heir to the "Peanuts by Planter" farming company.

When Grim cannot take any more abuse from Mandy, Billy tries using Grim's scythe to shrink her to mouse size. Instead of shrinking, Mandy becomes a giant and destroys Billy's house. Although Mandy enjoys being a giant for a while, there are inconveniences, as she keeps growing as time passes, and she wants to return to her normal size. Billy is the only one who can cancel the curse he put on Mandy, and he refuses to do so. Mandy grows bigger than the planet, and she finds herself in the "Realm of the Infinite" with other creatures that grow at infinite pace. Mandy starts bossing them around to please her.

When Billy's dad drives Billy, Mandy and Grim to the hamburger restaurant, they somehow end up in Mexico. There, the people are celebrating Dia de los Muertos, but local kids say it may be the last celebration ever, because Del Uglio, a masked and ugly luchador, is set on stealing the "Golden Nose of Chipotle", a prehispanic relic on which the festivity is based. To get to the nose, Del Uglio has one half of the map, while the other half is held by his lost twin brother, who turns out to be Billy. Grim defeats Del Uglio, but it is Mandy who keeps the golden nose for herself, which makes her rich.

When Billy starts taking pictures with a camera found in Grim's trunk, Grim tells him that is an Underworld's True-heart Camera, which shows the real essence of a person's heart. When Mandy takes Irwin's picture, it is revealed that he is actually half-good and half-evil. Irwin explains that although he was born evil, his dad taught him about love and respect, and he had a change of heart. Upon knowing this, and against her own wishes, Mandy also experiences a painful change, turning good and kind-hearted. Mandy and Irwin finally share a romantic kiss in this episode, but it makes Mandy revert to her old dark and sinister self, which was Irwin's true intention, as he likes her just the way she is.