The protagonist of the story is a shy employee of a brokerage firm named Yuk Tong-shik. One day, he accidentally comes across the diary of a serial killer, in which the criminal describes his crimes in detail. After a tragic incident, the young man loses his memory and believes that he himself is the culprit. From that moment on, Tong-shik undergoes a rapid transformation. Detective Shim Bo-gyeong from the law enforcement agency wants to catch the real criminal and believes that the main character can help her with this.