For so many years, there has been one thing that has made Choi Sang-kyu (played by Jung Jae-young) unable to let go. It was 17 years ago when an extremely serious series of murders of women took place in society, and Choi Sang-kyu was the detective in charge of the case. The cunning murderer cleverly evaded the nets of justice. On the day the 15-year statute of limitations arrived and the murderer completely escaped legal punishment, the victims' families issued a plea against the incompetent police and the brutal killer. Just when everyone thought the shadow of the tragedy was about to dissipate, the publication of a book titled 'I Am the Murderer' shocked everyone, and the author of the book was none other than Lee Doo-sik (played by Park Si-hoo), the instigator of that murder case. However, as the detective who pursued the murderer, Choi Sang-kyu does not believe that Lee Doo-sik is the criminal from that time, leading to an intense cat-and-mouse game between the two.