Tae Seok is a detective who's tormented by a pang of guilt that he missed the culprit that murdered a high school girl named Yeo Wool in 2005. Now Tae Seok meets with a prosecutor, and he turns out to be a suspect who killed Yeo Wool in 2005. Meanwhile, a housewife gets abducted, Tae Seok starts to investigate.

With a strong belief that Prosecutor Jang Hyun Min is the abductor, Tae Seok starts to look into Hyun Min’s background and past. When he sneaks in Hyun Min’s house, he’s reminded of when Yeo Wool and Soo Ah ended up being killed and the police failed to prevent the murders in 2005. After relentlessly pursuing the culprit, Tae Seok finally arrives at the crime scene where he finds the abductee.

Prosecutor Jang Hyung Min who fell from the stairs gets taken to a hospital, but he remains unconscious. The police higher-ups discuss whether to punish Tae Seok for letting Hyung Min fall and get severely injured. However, Tae Seok has already decided to resign. Meanwhile, Seon Jae's parents get murdered and Eun Jae calls the police.

Tae Seok continues to suspect that Reporter Eun Seon Jae is a psychopath killer who murdered her parents and dog. Rather than getting frightened or denying, Eun Seon Jae seems to enjoy getting suspected and even shows off how clever she is. And as Tae Seok keeps provoking her, she threatens someone precious to Tae Seok.

Tae Seok finds Yeo Wool's name tag from Seon Jae. He questions how she knows Yeo Wool, but Seon Jae only gives a vague answer. Tae Seok starts to look into the relationship between Yeo Wool and Seon Jae and who Seon Jae really is. Meanwhile, the police receive a new murder case where two officers have gotten killed.

Seon Jae tries to poison Song Man Soo who is in prison now and asks him why he killed Yeo Wool in the past. He says it was Jang Hyung Min who killed Yeo Wool, and not him. In the meantime, the man who murdered two police officers shows up again, and this time, he shoots more officers with a gun in front of Tae Seok.

When Tae Seok arrives at the hospital, Jang Hyung Min is awake and still remembers that Tae Seok let him fall from the stairs. In the same room, Seon Jae is expecting Tae Seok to come. Meanwhile, the police team led by Jeon Chun Man falls into the trap where Song Yoo Jin has planted a bomb.

Jang Hyung Min plans to reveal what Tae Seok and the police did to him to put Tae Seok in a pickle. And even a press conference is held to question Tae Seok. In the meantime, Song Yoo Jin escapes from the police and plans another terror attack. And Seon Jae comes to visit Hae Jun unannounced and makes a threat again.

Tae Seok continues to chase Hyung Min, who is receiving help from Chun Man. They end up cornering him at a shopping mall but he manages to slip away. Right before having Hyung Min on his hands, Tae Seok gets caught by Chun Man. Chun Man puts handcuffs on Tae Seok’s hands, claiming that he is the culprit of Hae Jun’s case.

Young Hyun Min, known as Seon Ho, encounters Tae Seok’s little sister and her friend. The two high school girls witness a brutal murder that he commits, but Seon Ho knows that they can’t just go to the police for help. Meanwhile, Tae Seok points Yeo Wool as a witness instead of Tae Hee after coming to know that Tae Hee was the real witness.

Tae Seok seems to be trying to hide his sorrow over Hae Jun's death, and his colleagues console him. In the meantime, another murder takes place, and it seems to be done by Jang Hyung Min who is known to have died from a car accident. Tae Seok's team starts to investigate the murder and looks into the killing method to find out if it's the same as Jang Hyung Min's.

Following a hunch, Tae Seok finds out who the masked serial killer is. As soon as he spots the killer who has shown up again at the scene, Tae Seok starts to go after him. They end up in dark butcher storage, and Tae Seok suddenly gets attacked and collapses. In the meantime, the police call in a survivor from the previous case of the masked killer and ask her if she remembers anything.

Tae Seok tries his best to find Dong Yoon by not following the official manual. Dong Yoon remembers what Tae Seok said before, and tries not to provoke the murderer, Joo Seong. Meanwhile, Chun Man gets appointed as Captain of Dong Yoon’s case.

Detective Chae learns that there's one more woman that the masked killer is after. In the meantime, Tae Seok manages to find the traces that lead to the location where Detective Chae is locked in. When everyone is relieved by the fact that Detective Chae is alive, another murder takes place, and its method of murder is similar to Jang Hyung Min's.

Seon Jae gets arrested as a suspect for murdering Go Joo Seong. She tells that Jang Hyung Min is the one who killed Go Joo Seong and she saw him for himself, but the police, except for Tae Seok, wouldn't trust her because there's no evidence. In the meantime, Jeon Joo Hyun who ran away from the murder scene ends up in a car accident.

As Chun Man suggested, Tae Seok steals diamonds from the chief's safe. Tae Seok and Chun Man go to deal with the smugglers, but things go south. Tae Seok manages to rescue Jeon Joo Hyun, but the smugglers take the diamonds. Meanwhile, thanks to Jeon Joo Hyun's statement, Eun Seon Jae is freed from the police.

Another woman is murdered by Jang Hyung Min and strangely this time, his fingerprints are found everywhere as if he’s provoking the police. The killer leaves behind a photo of a woman. To prevent another possible murder, the team needs to find the woman in the photo or find the killer first.

Jeon Chun Man tells Jang Hyung Min to end their relationship, but Jang Hyung Min insists that Jeon Chun Man should continue to share the inside information of the police. Meanwhile, following the evidence, Tae Seok’s team manages to find the place where Jang Hyung Min is likely to be. When they arrive there, they find a crime scene that reminds Tae Seok of a murder case in the past.

Jang Hyung Min narrowly runs away from the police with the help of Jeon Chun Man. Knowing that Jang Hyung Min is after Tae Hee, Tae Seok brings her to his house for her safety. Meanwhile, Eun Seon Jae goes to her parents’ house to see if there’s anything that can help regain her memory.

Detective Chae is told to tail Jeon Chun Man, but he soon gets caught. And Jeon Chun Man gets into the vault without being caught. In the meantime, Tae Hee gets abducted by Jang Hyung Min. He tells Eun Seon Jae to come over to see Tae Hee. While Jang Hyung Min is away, Eun Seon Jae tries to help Tae Hee get away.

Seon Jae and Tae Hee are kidnapped by Hyung Min, and Tae Seok tells Hyung Min to kill Seon Jae. Even though Hyung Min is killed at the firearm incident and the rest are safe, Tae Seok is still confused. Meanwhile, Seon Jae comes out of a coma and she goes to Tae Seok

The Punisher has captured another criminal and started a broadcast. He wishes the public to decide the criminal’s fate by vote. The only way to save the criminal, Jang Seok Goo, is to ask his victim, Cha Na Ri to forgive him

Tae Seok and his team members locate where Punisher and Jang Seok Goo are and manages to rescue Jang Seok Goo. Punisher runs away and Tae Seok goes after him. Based on what Punisher has told about his mistrust of the police, Tae Seok tries to figure out Punisher's motive and the next target.

Tae Seok plans to use Kwon Soo Chul as a bait to catch Kang Woo Joon. On the day Kwon Soo Chul is discharged from the prison, Tae Seok and Detective Chae take Kwon Soo Chul to a safe place. Meanwhile, Eun Seon Jae asks Tae Seok to catch the culprit who killed her parents.

A teller has been murdered but it was no ordinary crime. It is a murder that is similar to a serial murder case that was unsolved for 20 years. Whether it is the same culprit or not is up to the team to investigate.

The serial killer from 20 years ago has revealed himself to the police. However, due to the statute of limitations, the police cannot arrest him. The problem now is to figure out who the recent murder was done by.

Lee Sung Hak says he didn't kill Kwon Ji Sook, and there was another witness at the crime scene back in 20 years ago. The police continue working to find evidence of the murder case. And the police come across a man who is possibly involved both in the murder from 20 years ago and the recent one.

Tae Seok and his team members catch Yoo Jung Soo at a crime scene and start to question him. Meanwhile, Tae Seok and Mun Gi continue to work to find out if Eun Seon Jae really killed his parents and if there was an accomplice. And they get a CCTV footage of Eun Seon Jae making a purchase in a store.

Manager Seo visits Tae Seok to give his confession. Manager Seo claims he was the culprit for the murder of Eun Hong Goo and Park Jung Sook. Tae Seok and his team believe it wasn’t him, but he knows too many details that only the culprit would know.

Ms. Hwang, who has lost her son, is being accused of murdering the hospital director. She is fully aware of what happened to the director but refuses to talk until Tae Seok finds out what happened to her son. Meanwhile, Chun Man is trying to save himself from this case.

Seo Jae Geun tells Tae Seok everything from how he got involved with Eun Seon Jae and her foster parents to how he has recently ended up with making a false statement. Meanwhile, Detective Chae is asked to wear a microphone without telling his team members so that the Internal Affairs Team can secretly record and investigate what Tae Seok is up to.

Tae Seok tuns on the recorder that reveals the fact that Jeon Chun Man helped Jang Hyung Min for a long time. Humiliated by Tae Seok In front of other police officers, Jeon Chun Man tries to take the recorder away from Tae Seok. Meanwhile, Eun Seon Jae goes missing. Tae Seok manages to locate her and goes to meet her.