Yook Dong-sik is a timid, pushover employee at a brokerage firm who is constantly mistreated by his family and colleagues. Driven to the brink of despair, he attempts suicide but instead witnesses a gruesome murder committed by a serial killer. While fleeing the scene in a panic, he is accidentally hit by a police car and suffers severe amnesia, losing all memories of who he is. However, he clutches the killer's diary—which details all of the murderer's sadistic crimes—and, unable to read his own past, convinces himself that he is the actual psychopath who wrote the journal. Bolstered by this chilling misconception, Dong-sik undergoes a drastic personality transformation, mistaking his newfound delusion for inner strength and beginning to act like a cold-blooded predator against those who once bullied him. Meanwhile, the real serial killer, Seo In-woo, happens to be a director at Dong-sik's company, leading to a tense, darkly comedic cat-and-mouse game. Complicating matters further is Shim Bo-kyung, a passionate community police officer determined to catch the serial killer using only her instincts and intellect, who crosses paths with the newly confident Dong-sik.