Yook Dong-sik (Yoon Shi-yoon), an analyst who works at Daehan Securities, has a timid personality.

One day, shortly after failing on his attempt of suicide, he witnesses a murder.

He flees with the diary, but he is accidentally hit by Police Officer Shim Bo-kyung's (Jung In-sun) patrol car which causes him to suffer from retrograde amnesia.

Due to the diary in his possession, Dong-sik mistakenly believes he is a psychopath.

Meanwhile, Seo In-woo (Park Sung-hoon), the director and son of the chairman of Daehan Securities where Dong-sik works, turns out to be the heartless psychopath serial killer.