“Psychopath Diary” is about a man named Yook Dong Shik, who coincidentally witnesses a murder and loses his memory after an accident that occurs while fleeing the scene. He wakes up with no sense of identity and mistakes himself as the killer when he finds the murderer’s diary in his hands.
To find evidence pinning In Woo to Chairman Seo Choong Hyun's murder, Dong Sik devises and starts a new plan. While Bo Kyung hooks In Woo up to a lie detector for further questioning, Dong Sik and Chil Sung search In Wook's place and discover his hideout. Meanwhile, Dong Sik's colleagues at Daehan Securities post rumors regarding the mystery behind the chairman's murder.
Dong Sik's memory returns to him with Bo Kyung's help. When he realizes In Wook is the serial killer playing him as the fool, rage takes over him. Finally, Bo Kyung and Chil Sung help Dong Sik start to make his move to turn the tables on In Wook.
In Woo tries to get close with Dong Sik after secretly following him and seeing him kidnap Ji Hoon. In Woo becomes the director of the Asset Management Team and promotes Dong Sik to assistant manager. Dong Sik later finds out that he had written a suicide note on the day of killing the homeless man and is thrown into confusion. Meanwhile, Bo Kyung, who is after the serial killer, starts to find Dong Sik's behavior strangely suspicious.
Dong Sik and Bo Kyung try to uncover the truth of a murder that happened eight years ago. As the two visit the crime scene, Dong Sik tries to convince himself that it wasn't him. Meanwhile, In Woo plans to set Dong Sik up as the killer to all his past murders, and Bo Kyung receives an anonymous text message about the predatory serial killer.
Bo Kyung digs into her uncle's death and finds evidence proving Dong Sik's innocence. However, she falls victim to In Wook's watchful eyes and finds herself in a dire situation. Meanwhile, in prison, Dong Sik gets put on disciplinary action, and without any time or choice, he makes a daring escape.
Being the last person talking to Sung Chul, Dong Sik tells the police that Sung Chul is the predatory killer they are after. The team decided to close the case although Bo Kyung believes that he could be just a copycat killer, and the real predator still hasn’t been caught. Meanwhile, In Woo learns the truth that Dong Sik is suffered from amnesia due to the accident.
Dong Sik determines to kill Seo Ji Hoon and secretly plans how he's going to do it. In Woo tells Dong Sik about a company going public and urges him to buy as many shares as he can. Meanwhile, Ji Hoon and Chan Suk resort to the old-fashioned way to get rid of Dong Sik, and Bo Kyung goes on a suspicious call.
Bo Kyung and In Woo both chase after Dong Sik for different reasons. And soon, both get closer to what they are after. Bo Kyung gets an unexpected hint from Dong Sik about the serial murders and starts to investigate a missing person case. Meanwhile, In Woo starts to become more active.
It's the final battle between Dong Sik and In Woo. With the police heading their way, Dong Sik fights off In Woo, who has a gun pointed at Bo Kyung's face. As the police arrive, gunshots echo down the emergency stairway. Who will be the victor? In Woo, the psychopath serial killer? Or Dong Sik, the pushover?
Dong Sik, a stock analyst at Daehan Securities, takes the blame for a report he didn’t write that caused a leading pharmaceutical company’s stock price to fall. He decides to end his life and attempts to jump. However, a murder about to happen catches his eye.
Another death made to look like a suicide arises and puts Dong Sik and In Woo on edge. According to the victim's husband's statement and with a composite sketch of the killer, Bo Kyung and the Predator Killings Task Force head out in search of the suspect. Later, someone claiming to be the serial killer calls a broadcasting network.
Dong Sik wants to stop Bo Kyung from looking into the murder case happened at Goseong Chemical, so he follows her and offers help. Dong Sik gets to know Nergui by chance when he sneaks into Goseong Chemical. Nergui asks for Dong Sik’s help and tells him that Jin Goo doesn’t kill himself. Meanwhile, an open investigation begins after a dead body is found in Goseong Chemical, and In Woo finds out that Dong Sik and Bo Kyung actually know each other.
Bo Kyung arrests Dong Sik as the predator serial killer, and Dong Sik stands in front of the press for pictures. However, after Dong Sik gets sentenced to life in prison, Bo Kyung comes across evidence that might prove Dong Sik's innocence. In prison, Dong Sik meets Oh Myung Dal, who was wrongfully accused of Son Ji Min's murder eight years ago, and during a chaotic moment, his real identity flashes before his eyes.
In Woo has Dong Sik right where he wants him. Dong Sik has no clue that In Woo is the one behind the blackmail and prepares for murder. Meanwhile, Bo Kyung, who thinks former police officer Park Moo Suk is the informant, finds his body.
Bo Kyung starts having doubts about Dong Sik being a nice guy and attempts to get a sample of his DNA. Dong Sik, who is agonizing over the fact he killed Park Moo Suk, returns the million dollars to In Woo and decides to leave town. However, as Dong Sik leaves town, he soon realizes that he left the red diary at the restaurant he was at with In Woo.
After the restroom incident with Mr. Gong, Dong Sik becomes jubilant. His world of being a pushover ends, and he begins to believe that he is indeed a predator. Meanwhile, Seo In Woo, the serial killer, starts to look for his missing diary, and Shim Bo Kyung, a patrol officer, discovers something strange in the violent crimes and attempts to find the homeless man who vanished.
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