Kim is a widowed drug policeman who meets with a potential informant named Kang. While they were talking at a restaurant, Kang was arrested by the murder department for killing his girlfriend. He confessed to the murder and was brought behind bars.
A few months later, Kang contacted Kim from the prison and claimed that he had murdered 6 other people. However, he refuses to speak to murder detectives, whom he claims is fabricating evidence against him in order to put him in jail. Kim agreed to find evidence in exchange for information on the remaining six bodies. Kim discovers real evidence with Kang's information proving that the murder detectives fabricated his case. As a result, detectives are charged and Kang gets a reduced sentence. In return, Kang provided information about the remaining 6 victims as promised. Among the remaining six victims, Kim first investigated the disappearance of Oh Ji Hee, a former swimmer forced to work at a nightclub to support her grandmother in the village. Kang claimed he killed her one night after taking her out of town while working as a taxi driver there. During the investigation, Kim was repeatedly told by people he questioned about Kang that he was unreliable. However, Kim persisted. With much difficulty, Kang found a part of Oh's remains. However, when questioned by Kim, Kang lied that Kim forced him to make false statements and bribed him to do so. Furthermore, the DNA results showed that the remains were not of Oh. The prosecutor cannot charge Kang because the evidence against him is not convincing enough.
Kim felt deceived and turned to his former mentor, Song, for advice. Song advised Kim to stop pursuing Kang's case because the murderer is trying to make himself seem innocent in other crimes to overturn the sentence for the crime he is currently serving. Kim also visited Kang's sister, who revealed Kang killed their father when he was just a boy. She helped him cover up the murder. As a result, Kang grows up to be a serial killer who is responsible for many other deaths. Kim was unable to take this case to court because the statute of limitations had ended.
With the help of his new partner Jo, Kim begins investigating the second victim, Hwang, whom Kang claims he killed by pushing him down the stairs. Kim and Jo examine old files of unsolved cases and find a case that matches the Hwang murder case. The case went to trial, but because there was only evidence, Kang was not charged with murder. As a result, Kim was demoted and Jo transferred to the drug department.