Major crimes unit detective Seong-jin (SON Hyun-joo) arrests Tae-sung for petty crimes, but shocked to find out that he is the notorious serial killer. However, Seong-jin becomes doubtful of his identity as the serial murder case is investigated. Nevertheless, when Gyu-nam (JANG Hyeok) offers help to treat Seong-jin's sick son, Seong-jin has to assist him in fabricating the evidence. Meanwhile, journalist Chu dies under suspicious circumstances, and Seong-jin realizes that Gyu-nam is behind Chu's death and tries to find out the truth. But, as Seong-jin gets closer to the truth, Gyu-nam's threats intensify.