Yook Dong Shik works at a brokerage company. He has a timid personality and he can’t even get angry at the people who look down upon him. One day, he witnesses a murder. He accidentally picks up a diary belonging to a killer, who is a serial murderer. The diary has a log of his murders. Yook Dong Shik flees with the diary, but he is accidentally hit by Police Officer Sim Bo Kyung’s car. Yook Dong Shik loses his memory because of the accident. Due to the diary in his possession, Yook Dong Shik mistakenly believes he is a serial killer. Since that moment, he begins to change. Sim Bo Kyung, who got involved with Yook Dong Shik, decides to catch the serial killer. Meanwhile, Seo In Woo works as a director at the brokerage company where Yook Dong Shik works. He is heartless and a psychopath serial killer. He has lost the diary in which he detailed his crimes. He now feels insecure.
