Se -hyun, who visited the body organic field with Chung Hyun, recalls the traces of his dead murderer, Yoon Jo Kyun. Meanwhile, after visiting Yongcheon site, Oh Min -ho, the team member of Se -hyun, continues the absence for a few days.

In Yongcheon, a carcass with a thread is found, and the identity of the body is found to be the missing team of Sehyun. Sehyun, who discovered the same technique and clues in the second body, feels that the bacteria are back.

Chung Hyun, who arrested an innocent citizen as a suspect, is hit by the media, and Se -hyun, who helps him, announces a serial murder. Sehyun, who reveals his existence to Cho Kyun, tries to stimulate him and pull him over the surface.

Sehyun installs a temporary autopsy in Yongcheon and chases traces of the bacteria. That night, Sehyun finds a third carcass in front of his studio. Sehyun tells Cho Kyun, who celebrates his birthday, and tells him to start playing in the future.

Sehyun finds his childhood photographs planted by Cho Kyun in the third body and is shocked. Se -hyun, who is in a hurry by the caught clue, heads to Hae -ryong, who lived with Jo Kyun in the past. There, there is a faces of Cho Kyun and his new murder assistant, Se -eun.

Jung Hyun's investigation expands to the “Cheongwoon Port Incident”, where Se -hyun and Cho Kyun were together. In the investigation situation, Se -hyun suggests that Cho Kyun is making a fake criminal. However, Sehyun falls into a trap from the address sent by Cho Kyun.

Sehyun, who managed to deal with the trap of Cho Kyun, meets Jocyun at the appointment. On the other hand, Jung Hyun finds out that Se -hyun hid the outcome of the drug and doubts.

Jung Hyun is confused by the suspicious circumstances of Se -hyun, and Cho Kyun makes a full war to isolate Sehyun. Se -hyun, who has been seized and searched for the sin for deceiving the result of the drug, chooses silence in front of Jung -hyun, who asks for the truth.

Sehyun is a suspect of all murder in Cho Kyun's careful manipulation, and even Hyun -hyun pursues Sehyun. Se -hyun, who is at a corner, heads to a nursing hospital with a mother -in -law, and finds the situation where Cho Kyun went there. Sehyun decides with the last battle and heads to the residence of Joseun.

An attempted murder occurs at a laundromat, and Se-hyeon is wanted. Jeong-hyeon, who has been suspended from duty, decides to conduct an independent investigation. Meanwhile, Se-hyeon, who was captured by Jo Gyun and imprisoned in a truck, encounters Se-eun, the girl who replaced him, and gets caught up in a horrific family game.

Jeong-hyeon, who got closer to the truth through the living victim, tracks down Jo Gyun's truck. Se-hyeon and Se-eun devise a way to escape from the truck.

Jung-hyun is in danger due to a serious injury. Se-hyun is forced to make a crucial decision to protect Se-eun, and let the world know that there is another victim, Se-eun, who is still suffering.

Se-hyeon confesses his past to Jeong-hyeon. The two decide to work together to capture Jo Gyun. Meanwhile, Jo Gyun threatens Se-hyeon using Se-eun's life as an excuse.

With Se-eun's whereabouts unknown, Se-hyeon becomes enraged, believing that Jo-gyun has already killed the girl. After receiving Jo-gyun's message, Se-hyeon escapes the hospital and tracks down Jo-gyun. Jeong-hyeon sets out to find Se-eun.

Despite his injury, Jeong-hyeon does not give up on finding Se-eun to keep his promise to Se-hyeon. At the same time, Se-hyeon faces Jo-gyun at the Mujin Tidal Flat and is faced with a final choice.