Psychopath Diary is a South Korean dark comedy thriller about Yook Dong-sik, a timid office worker who accidentally witnesses a murder and loses his memory while fleeing the scene. When he wakes up with a serial killer's diary in his hands, he mistakenly believes he is the psychopath. His personality slowly changes as he tries to "fit" the role, while the real killer, a cold-hearted stockbroker, hunts him down to retrieve the diary. It's a mix of suspense, comedy, and psychological twists that keep every episode unpredictable.