Kang Tae Joo returns to his hometown, Kangseong, after being demoted following his chief's arrest. On the first day at the Kangseong office, he finds out from his colleagues that an ex-convict from his past case has been beaten. Although he believes he is not the culprit, the situation doesn’t go as he thinks. While investigating the case, he meets his classmate, Cha Si Yeong, who has bullied him badly. Si Yeong greets Tae Joo with a smile, but it seems Tae Joo hasn’t moved on from the memory.

After an uncomfortable reunion with Si Yeong, Tae Joo resumes tracking the serial killer. Ji Won, a female reporter, also joins the operation to catch the murderer. Meanwhile, Gi Beom proposes to Sun Yeong, but soon grows uneasy after seeing her leave work in her male colleague’s car. At the same time, Tae Joo, dispatched to a college student protest, spots a male student who resembles Gi Beom and begins to grow suspicious.

After Tae Joo used violence against the county governor’s nephew because Sun Yeong assaulted him, he is in danger of being resigned. Through the testimony from a victim, he realises that there is another victim of the serial murder case among the missing people. He promises that he will find the body of the new victim with the condition of his resignation. Meanwhile, Ji Won prepares a surprising event for Tae Joo’s reputation, and Sun Yeong tells Gi Beom that she wants to break up with him.

After the death of Min Ji, who showed trust in Tae Joo, Tae Joo meets Si Yeong and suggests catching the serial killer together. Tae Joo’s comeback arouses controversy among his colleagues, but he just focuses on tracking the killer. He discovers a connection between Min Ji and Gi Beom. To make things worse, Gi Beom becomes the suspect because of the assault on Jeon Kyung Ho, and the doubt toward him becomes stronger.

As suspicion toward Gi Beom grows, Tae Joo is confused because the circumstances point to Gi Beom as the killer, but he hesitates to arrest him for some reason. After another murder case happens, he tries to find the missing Gi Beom to ask about everything, being sure that Gi Beom is the killer. Si Yeong, who was attacked by the killer, wakes up, and he reconciles with Tae Joo. But with the emergence of Si Yeong’s brother, the situation of the investigation unit becomes complicated.

Gi Beom is continuously confined, tortured, and threatened, forced into giving a false confession by Si Yeong and his subordinate detectives. Using a handbag found as evidence, Ji Won begins to deduce the real culprit and eventually tracks down a suspicious man, attempting to interview him. Meanwhile, Tae Joo seeks out a nuclear research scientist, trying a new method to track down the perpetrator.