On their first day as partners, detectives Kang Do-chang and Oh Ji-hyeok pick up a series of calls with perplexing connections to a closed murder case.

Words get out of Incheon Seobu Police Station revealing that Lee Eun Hye, the daughter of a murderer Lee Dae Chul, has been kidnapped, murdered, and buried by a man who claims to be the culprit. As the news goes viral, people who were related to the murder case from five years ago tremble in fear of possibilities of getting brought to justice.

The execution of death-row convict Lee Dae Chul nears as well as the evaluation for Do Chang’s promotion. Knowing that Lee Dae Chul has an alibi at the time of the murder, Do Chang needs to decide whether to save an innocent man or to submit to the authority. Meanwhile, Do Chang and Ji Hyuk are joined by Seo Gyeong at Dae Chul’s old workplace, and that’s when Seo Gyeong gets a phone call from Park Gun Ho, who seemed to sound very urgent.

A big body of corruption sweeps over Violent Crimes Unit Two with the higher-ups continuously pressuring them to turn a blind eye to Lee Dae Chul’s case. Meanwhile, Ji Hyuk saves Seo Gyeong from Jo Seong Dae who is suspected of killing Park Gun Ho. Eventually, clues that lead to Lee Dae Chul’s innocence start piling up as Do Chang decides to look into the case to clarify discomforting ambiguities.

While clues proving a corruptive body’s close relation to Lee Dae Chul’s case pile up, Do Chang figures out a work-around to carry out a secret investigation on Jo Seong Dae. On top of that, Ji Hyuk hints to him how his very cousin, Oh Jong Tae, may be partially responsible for Yoon Ji Sun’s death from five years ago. Meanwhile, Dae Chul’s day of execution nears as his daughter, Eun Hye, visits him at the prison.

Do Chang confesses to Eun Hye that he wants to save her father, Lee Dae Chul. Unfortunately, Eun Hye can’t trust Do Chang who literally handcuffed and arrested her father for a murder he didn’t commit. However, Do Chang shows a sign of hope for Eun Hye by getting the re-trial approved. Meanwhile, Superintendent Moon and Yoon Sang Mi prepare themselves for the possibility of an unexpected turn of events.

Although Do Chang and Mr. So prepare for the retrial of Lee Dae Chul’s case, Do Chang still needs to persuade Dae Chul to follow his lead because he is reluctant to take part in it due to the possibility of harm that it may cause his daughter. Meanwhile, Do Chang finds himself caught in a dilemma that is inflicting harm to both the victims who lost their beloved partners and the ones who crossed the line for their greed.

It was already difficult enough for Do Chang and others to get a retrial approved. However, feeling desperate during the trial, Dae Chul falsely admits to his killing of Detective Jang Jin Soo. Meanwhile, Seo Gyeong receives a recording from an unknown sender of a conversation between Superintendent Moon and a prosecutor, which could act as solid evidence to prove Dae Chul’s innocence.

After losing at the retrial, Do Chang gets demoted. With Do Chang out of the picture, Violent Crimes Unit Two also feels pointless to even fight for a comeback. Meanwhile, Ji Hyuk meets Seo Gyeong and finds out about a voice recording that she’s been keeping secret. Ji Hyuk and Do Chang then indirectly threaten Superintendent Moon so that they can follow him and see where he would lead them.

Do Chang gets reinstated and returns to Violent Crimes Unit Two after coming to an agreement with Superintendent Moon. The team then focuses on Park Gun Ho’s case and stumbles upon Jo Seong Dae’s bloodstain at the murder site. This leads to the revelation of Chief Nam's wrongdoing as well as a possible connection to Oh Jong Tae. Just before closing in on Jo Seong Dae, the team catches the sight of the one behind the corruption.

With the help of a little stunt pulled off by Jae Hong, Do Chang gets one step closer to proving Oh Jong Tae’s crime. Meanwhile, Ji Hyuk barges into Yoo Jeong Seok’s office, asking if he is closely acquainted with his cousin, Oh Jong Tae. Even with all evidence closing in on him, Oh Jong Tae still holds what seems to be the key to shaking off all things that annoy him.

While Ji Hyuk manages to survive the ambush by Oh Jong Tae, the prosecution adds more pressure to Do Chang and his colleagues, invalidating their efforts to bring Oh Jong Tae down. Do Chang however continues to put pieces together and stumbles upon a crucial link that connects Oh Jong Tae to his evil deed. Meanwhile, Seo Gyeong finds out that her boss, Yoo Jeong Seok, was at the murder scene five years ago.

Jin Seo Gyeong hosts a secret meeting with Yoo Jeong Seok and Kim Gi Tae at Gawol Church, demanding them the truth regarding a missing person named Jo Seong Gi. In the meantime, Do Chang and Ji Hyuk successfully locate the missing dead body from five years ago. Despite the evidence against their favor, Oh Jong Tae and Nam Guk Hyun cook up a loophole to clear the charges.

Even after the arrest, Oh Jong Tae still has the audacity to claim his innocence and goes on to emphasize the incompetence and corruption of the police organization. Meanwhile, Seo Gyeong and Sang Mi sidestep to approach the matter in their own way. Eventually, Do Chang reaches Yoo Jeong Seok with a warrant, and Jeong Seok shocks everyone by uttering a short confession.

Yoo Jeong Seok admits to his doings before his junior reporters and leaves the office, ordering them to stay put until his next instruction. While being threatened by Oh Jong Tae, Nam Guk Hyun offers video footage of Jeong Seok’s murders. On the verge of being exposed, Jeong Seok willingly suggests the police that he would confess in his unique manner. The confession is officially published.

The shocking death of Yoo Jeong Seok stirs up the public, and the case is now under a spotlight. As the prosecutor makes a confession to his previous wrongdoing out of conscience, Oh Jong Tae gets sentenced to a lifetime in prison. However, Ji Hyuk’s gut feeling tingles down his back, and he starts searching for something. Something that might just reveal the truth behind the death of a fellow detective, Jang Joon Gi.

A serial killer is on the loose. As Do-chang battles past trauma, the investigation grows murkier, only taking him further from the truth.

Do-chang’s guilt grows after another unexpected death takes place. Others want to wrap up the case, but Do-chang and Ji-hyeok refuse to give up.

The detectives find some strong evidence and start going over CCTV footage. A man burns mysterious objects, and makes a suspicious approach.

Do-chang and Ji-hyeok are hot on the killer’s heels, but elsewhere another murder is underway. Cheon Sang-u keeps an eye on Cheon Na-na.

Do-chang finds out about the killer’s tragic past and realizes that one piece of his pattern of murders doesn’t line up. He induces a confession.

Na-na follows Sang-u and finds some evidence. The team is threatened with being broken up. The detectives investigate Jung Hui-ju’s acquaintances.

Kim Min-ji’s background is revealed. The detectives get proof that incriminates TJ Group executives. Ji-hyeok is curious about Na-na’s intentions.

Na-na’s press conference throws Woo Tae-ho into disarray. A suspect in the Hui-ju case is caught and some clarity is brought to the case.

The detectives question Na-na about the car accident. They investigate Tae-ho's previous whereabouts, but Choi Yong-geun is looking into this too.

Na-na preys on Sang-u’s weaknesses. A mysterious man follows Moon Bo-kyeong, while Yong-geun plans to use her connections to his advantage.

Yong-geun is arrested, but uses his experience to make the interrogation difficult. The detectives search for a motive for Sang-u’s murder.

Sang-u is furious after realizing that Na-na came close to turning him in. Ji-hyeok provokes Yong-geun in an attempt to make Sang-u culpable.

The detectives ask to reopen Tae-ho’s case and look for further evidence regarding his death. Na-na asks Ji-hyeok to make an allegiance with her.

The detectives question the gang who tried to kill Hui-ju’s grandfather. As they get closer to the truth, Na-na and Jang Ki-jin get into trouble.

Na-na passes evidence of her father threatening her to Ji-hyeok, but he uses it against her. Ki-jin files a false charge against Do-chang’s team.

While the detectives search for proof of Na-na’s crimes, she becomes chairman of TJ Group. Ji-hyeok gives testimony to get Sang-u released.