Yook Dong-Sik, a timid and easily overlooked employee, works at a brokerage firm. He is unable to stand up for himself and even fails to get angry at those who belittle him. One day, he witnesses a murder and by chance, he finds the killer's diary, which documents his serial killings. Yook Dong-Sik flees with the diary, but accidentally gets hit by the car of Police Officer Sim Bo-Kyung. Due to the accident, Yook Dong-Sik loses his memory. Mistaking himself as a serial killer due to the diary he possesses, he undergoes a transformation. Sim Bo-Kyung, who has a connection with Yook Dong-Sik, decides to capture the real serial killer.

Meanwhile, Seo In-Woo, the director at the brokerage firm where Yook Dong-Sik works, is a heartless and psychopathic serial killer. He has lost the diary that contains detailed records of his crimes, causing him to feel uneasy.