After the rape and murder of five elementary schoolgirls, the police have still failed to apprehend the serial killer. After one suspect, Yu Min-cheol (Kim Seung-hun), is shot dead — but with no conclusive proof he was guilty — the country's president becomes involved and adds to the pressure on the police to solve the case. Choi Cheol-gi (Hwang Jung-min), a brilliant but sidelined detective at the Metropolitan Investigation Services who has just brought down corrupt property developer Kim Yang-su (Jo Yeong-jin), is suddenly taken off the case and Kim, thanks to his powerful connections, is released and his case closed. Choi is assigned instead to the serial murders and finally promised a promotion if he can get the police force off the hook by bringing the case to a satisfying conclusion. Choi re-examines the whole case and, with the help of Kim's opportunistic rival, Jang Seok-gu (Yoo Hae-jin), decides to stitch up one of the other suspects as the killer. He chooses Lee Dong-seok (Woo Don-gi), a school bus driver with a retarded wife (Lee Mi-do) and young daughter (Park Ha-yeong) who has a past criminal record that includes child molestation. However, Choi and his team are secretly monitored by Joo Yang (Ryoo Seung-bum), a Seoul District public prosecutor in the pocket of Kim, who is looking for payback on Choi for bringing him down and losing a construction project to Jang. One evening, however, Kim is stabbed to death while playing golf with Joo, and Joo receives embarrassing photos of himself and Kim together. Suspecting that Choi has fitted up Lee in the serial-murder case, Joo makes his suspicions known to his superiors but cannot present any hard evidence. After Joo is again made to look stupid when Lee mysteriously hangs himself in his holding cell, he launches a blitzkrieg investigation into Choi's whole career and family, and a deadly war breaks out between them, with Jang playing both sides off against each other.