Over the years, there has been one thing that Cui Xiangjiu (played by Zheng Zaiyong) has been unable to let go for a long time. It was 17 years ago, when a female serial murder case with extremely bad influence occurred in society, and Cui Xiangjiu was the detective in charge of the case. The cunning murderer cleverly escaped the dragnet of justice. When the 15-year litigation period came, when the murderer completely escaped legal sanctions, the victim's family members died and issued accusations against incompetent police and brutal murderers. Just when everyone thought that the shadow brought by the tragedy was about to dissipate, the publication of a book called "I am a Murderer" shocked everyone, and the author of the book was none other than Lee Doo-seok (played by Park Shi-hoo), the initiator of the murder case that year. However, as a detective who was chasing the murderer that year, Cui Xiangjiu did not believe that Lee Doo-seok was the criminal of that year, and a fierce cat-and-mouse battle unfolded between the two.