Oh Hyun Jae used to be a top criminal profiler. He solved many cases with his skills, but an explosion, caused by a serial killer, ended the life of his fiancé. After the horrific incident, Oh Hyun Jae vanished and he now lives in seclusion. Meanwhile, Cha Soo Young is a police officer in a rural area. She has a special ability, which allows her to remember everything that she saw. Due to her special ability, she is selected as a rookie detective who will partner with Oh Hyun Jae informally.
Su Young tells Hyun Jae that she wants to join the RIU, and Hyun Jae asks her why she wants to join the RIU. Su Young becomes a temporary teammate of the RIU. RIU tries to find another victim who might be alive. Su Young gets into danger while the investigation and Hyun Jae helps her at that moment.
Ha Young gets to know that Hyun Jae saw The Guy’s face. Dong Sik and Hyun Jae encounter each other at the police station. Ha Young arrests Hyun Jae all of a sudden in oder to lock him up. Hyun Jae wants to know what Ha Young is hiding. Meanwhile, Ha Young interrogates Dong Sik.
Hyun Jae gets to know that Hyung Pil manipulated the evidence log. Dong Sik plants bugs in the police station. Hyun Jae tells Su Young to interrogate Yeon Ja. Hyun Jae tells the detectives in RIU Team One that The Guy is the police officer. Hyung Pil decides to kill The Guy himself. Meanwhile, Ji Min and Ha Young try to find The Guy’s identity secretly.
Hyun Jae asks The Guy who ordered him to kill I Su. The Guy counterattacks Hyun Jae, and Ha Young shoots him at that moment. Su Young notices that Hyun Jae used her to bring out The Guy, and she tells him that she’ll never see him again. Meanwhile, Su Young goes to Hongwoon and she gets to meet the boy named Hyun Soo.
Su Young remembers that the boy in the car with the doll was smiling. The Guy tells Hyun Jae to find him if he wants to know who wanted I Su dead. Dong Sik joins the RIU for the case regarding Sang Sik’s death. Hyun Jae gets to know that Yo Han is The Guy. Meanwhile, Gyeong Su regains consciousness.
Ha Young request for the NFS to do the autopsy for Seung Hwan’s body. The result of the autopsy says that there’s no trace of resistance, and Hyun Jae thinks Seung Hwan was overpowered psychologically. Hyun Jae wants Su Young to think of what she saw, and Su Young remembers everything. With Su Young’s memory, the RIU Team One starts finding the culprit who killed Seung Hwan.
A man in a black sedan keeps murdering young people who are just about to join the work force. RIU Team One finds a survivor and asks for her help. Unlike his usual self, Hyun Jae passionately helps out the survivor to remember the case. Meanwhile, the detectives are trying to find the secret behind “4789”.
Chan Ku dies in the cold storage, but Ha Young can’t find any peppermint candy in his body. Su Young is officially a member of the Regional Investigation Unit. Someone leaves a peppermint candy in Jun Seok’s car. Meanwhile, Hyun Jae says that the next target of The Guy will be Jun Seok.
Hyun Jae finds Soo Tak in Yuseong Psychiatric Hospital, but Soo Tak is not in the right state of mind. Soo Tak escapes from the hospital, and the police start finding him. Sang Sik is found dead all of a sudden. Meanwhile, Su Young finally finds the culprit who hit her mom with his car.
Hyung Pil orders Ha Young to take Hyun Soo’s case because he wants to use this incident as an excuse to fire her. Hyun Jae becomes an official consultant of RIU Team One. Hyun Jae and Su Young start their investigation and find out that Jung Woo is involved in this incident. However, Jung Woo keeps saying that he doesn’t remember anything.
Hyun Jae asks Ha Young why she told Dong Sik I Su’s name. Ha Young tells Su Young it is not her fault that she didn’t realize Dong Sik’s real identity. Ha Young plans to arrest Dong Sik. Meanwhile, Dong Sik disguises himself as Jung Woo Sik to meet Sa Gang who is in prison.
Su Young goes to Hyun Jae to tell him about the crime scene, but he tells her it’s too late. Hyun Jae tells Su Young to be careful since the culprit saw her face. Bo Kwang goes to Su Young’s place, and Su Young notices that someone was in her room. Meanwhile, Su Young thinks that Bo Kwang is suspicious and she follows him.
RIU Team One finds a witness to the murder case. However, the witness states Tae Sik is not the culprit. He provides a drawing of the murderer, and he looks similar to Choi Du Man. But when the team gets to arrest him, Du Man is already dead. The case becomes a bigger mystery. Who is the real killer?
Su Young finds Hyun Soo, and Na Hee gets arrested. Commissioner General is the one who was behind Hyun Jae. Hyun Jae officially returns to RIU Team One. Two murder cases happen in Haewol-dong, and RIU Team One starts to investigate the cases. Meanwhile, Tae Sik turns himself in saying that he killed the two people.
A murder case happens in Hongwoon, and Su Young is the first officer who sees the scene. Because of the rain, the scene has been damaged, but Su Young can memorize the scene in detail. Ha Young recognizes Su Young’s ability, and Ha Young brings Su Young to Hyun Jae. Meanwhile, a man comes to the police station saying that he is the murderer.
Sa Gang attacks Hyun Jae at the back, and Ha Young tries to stop Sa Gang. RIU Team Two arrives and says to Team One that they’ll take the case from now on. Hyun Jae takes Dong Sik to his hideout. Ha Young tells Su Young to stop Hyun Jae. Meanwhile, Dong Sik tells Hyun Jae that Hyun Jae is the one who killed I Su.