Yook Dong Shik is a 34-year-old office worker who is so weak and timid that he is used by everybody around him. After an incident, he becomes the scapegoat of his company, and so facing losing his job and his family turning their back on him, he decides to commit suicide. While doing so, he unexpectedly stumbles across a murder scene, but gets into an accident while running from the murderer. He wakes up with amnesia with the psychopath's diary in his possession. With no clues to his true nature other than the diary, and his fragmented memory lining up with the diary entry, he begins to believe that he is in fact a psychopath serial killer. 30-year-old neighborhood police officer, Shim Bo Kyung is investigating a suspected serial homicide. After a few run-ins with Dong Shik, she believes that he is an extremely kind man. However, after one too many coincidences, she starts to suspect Dong Shik might be more involved in the case than she thought. 36-year-old Seo In Woo is a cold and meticulous psychopath. On the day of his 7th crime, things go wrong and he loses his diary where he writes of all his daily life and detailed description of his murders. (Source: MyDramaList)