Yook Dong-sik (Yoon Shi-yoon) is a timid and quiet guy who works in a brokerage company. Due to this unassuming and timid personality, and not knowing how to stand up for himself, he is always taken advantage and bullied by his office colleagues. He does not know how to say no to them. However, this is going to change.
One day, he found himself being a witness to a grisly murder. He also stumbles into the murderer’s diary. The diary contains a record of all the horrendous murders that the psycho serial killer has committed. Terrified by the murder that he has just witnessed, he tried to flee.
As he was fleeing, he was knocked down accidentally by a car, driven by Sim Bo-kyung (Jung In-sun). Bo-kyung is a police office in the local police station. She is horrified and feels bad about the car accident that she is involved with Dong-sik.
Bo-kyung, plagued by guilt, decides to go into the life of Dong-sik. As she gets more involved into his life, she decides to find and arrest the serial killer.
Yook Dong-sik lost his memory due to the car accident. He does not know have any memories of his past. When he wakes up, he thinks that he is the psycho serial killer instead. This is due to the gruesome diary that contains the murder log that he has. Dong-sik starts to make some serious personality changes, to how the psycho serial killer is like.
The real psycho serial killer, Seo In-woo (Park Sung-hoon) is still at large. He works as a director at the brokerage company where Dong-sik works. He feels insecure now as he lost his diary that contains a detailed log of all his murders.
Did Dong-sik regain his full memory and realize that he is not the psychomatic serial murderer?
Did Bo-kyung manage to find the real serial killer, Seo In-woo and help save Dong-sik?
What moves did In-woo make after losing his deadly diary? Did he recover it back from Dong-sik? Was he arrested in the end?
How will Dong-sik’s life be like in the end? Will he get back to his old life?