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 demaged, 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.

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 requests 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.

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.

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.

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.

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.

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.

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?

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”.