Byeong-soo (Sol Kyung-gu) was a serial killer. He has secret memoir recounting his killings, stating he killed them because "they deserved to die for their own atrocities." His first victim was his father, who was abusing the rest of his family. He explains that he buried his victims in some bamboo woods. His last victim was an unknown woman, and as he was driving away, he got in a car accident and injured his head, resulting in Alzheimer's disease. He stopped killing after the accident.Byeong-soo, now a veterinarian, lives with his teenage daughter Eun-hee (Kim Seol-hyun), who cares for him and records his activities in a tape recorder for him to remember them. The former serial killer goes to poetry class, where everyone praises his poems on the murders. However, when he threatens to kill them, they don't take him seriously. A woman, Jo Yeon-joo (Hwang Seok-jeong), from the class develops a crush on him, irritating him. He does his best to drive her away, but like the rest of the poetry group, she always thinks his threats are jokes from his poems. Byeong-soo is also unlikely friends with a policeman Byeong-man (Oh Dal-su).There is a new series of killings and the doctor wonders if he has started killing again. The doctor has a rear ender accident with someone and there is unexplained blood at the scene. He sneaks some of it away as the other driver gets out. The driver says he hit a deer but Byeong-soo thinks he has found the psychopath killer. He follows the man and discovers a woman's body, and reports it to the police. Byeong-soo also gives the police a sample of blood. Despite his own findings that the blood was human blood, the police state that it was deer blood.The man is revealed to be a local police man Tae-joo (Kim Nam-gil), who starts dating Eun-hee, meeting her while searching for Byeong-soo. He forbids her from seeing the young cop. To protect Eun-hee, he fights to keep his memory and he plans to kill Tae-joo. However it becomes apparent that his memories may not be as reliable as he expects them to be after attacking Eun-hee, mistaking her for the woman who was his last victim.Byeong-soo's fears are apparently confirmed when he finds himself restrained to a wall, with Tae-joo hacking into his memoir. Byeong-soo attempts to reach his tape recorder to record Tae-joo's conversation, but apparently fails, barely managing to grab it and drops it. Tae-joo reveals he swapped the blood sample with deer blood, and threatens to kill Eun-hee unless Byeong-soo takes the blame for Tae-joo's crimes. Byeong-soo then wakes up, with no memory of what happened, and the entire incident is assumed to be a dream.Byeong-soo orders Eun-hee to go with his sister (Gil Hae-yeon) in a taxi and sends them off. Byeong-soo follows Tae-joo at night, and finds footage of Yeon-joo being held hostage. When he presents this evidence to the police, they suspect Byeong-soo himself is responsible. He later learns his memories have also made him delusional, as his sister had committed suicide after he killed their father. This proves he didn't send Eun-hee away with her aunt, and comes to think his daughter was killed by him instead. Byeong-soo gives up with his suspicions, and thinks he was actually responsible for Tae-joo's murders, especially when he remembers his annoyance and threats toward Yeon-jo. The police discover the bodies buried by Byeong-soo in the woods.Byeong-soo remembers who his last victim was: his wife, whom he killed after discovering she was having an affair with another man. At this moment Byeong-soo learns that Eun-hee is not his biological daughter. After injuring his head in the car accident, he arrives home in a rage. Byeong-soo prepares to strangle a toddler Eun-hee, but his head injury causes his memory loss to kick in for the first time. When the pain goes away, he doesn't remember his wife's affair, instead comforting a frightened Eun-hee.Back in the present and filled with guilt, Byeong-soo prepares to kill himself as he listens to the remaining recordings of Eun-hee on his tape recorder. However, he stops when another recording of Tae-joo confessing his crimes plays. Byeong-soo actually recorded Tae-joo's confession and threats to his daughter when he reached for the device. Byeong-soo realizes that he sent Eun-hee into a taxi with Tae-joo as the driver.As Byeong-man prepares a search for Byeong-soo, he receives a call from the latter. Byeong-soo says that Tae-joo has his daughter, and plays the recording of Tae-joo's threats to the policeman, convincing him of the truth. The two work together to find Tae-joo, and Byeong-man follows him to a cabin in the woods. Byeong-man hides in the cabin and sees that Tae-joo has kidnapped Eun-hee, but Tae-joo kills the policeman as he prepares to apprehend him. As Byeong-soo arrives to the location, he records that he must save Eun-hee, and replaces her picture with Tae-joo's in a locket she gave to him to remember that he is a killer.As Tae-joo is outside, Byeong-soo hides Eun-hee and Tae-joo attacks him. With his head injured again, Byeong-soo goes into a near-vegetative state, and he unwittingly leads Tae-joo to Eun-hee's hiding spot. As Tae-joo approaches Eun-hee, he reveals the truth of her father being a serial killer, and that he has to pay the price for his sins. Byeong-soo's memory restores and he manages to fight back before Tae-joo can kill her. Tae-joo appears to gain the upper hand, but Byeong-soo chokes him at the last minute and rolls on top of him, using the same technique he did to kill his previous victims. Tae-joo stabs him with a broken vase, but Byeong-soo persists. Finally resigning to his fate, the choking Tae-joo says that he himself will have to be punished. Byeong-soo takes the broken vase and stabs him to death. Eun-hee is now horrified by Byeong-soo, now knowing the truth about her father. However, he tells her that she is not the daughter of a murderer because of his wife's affair. Byeong-soo is later arrested. Eun-hee listens to Byeong-soo's latest recording about the need to find her and that she is the only reason why he is still alive. When she visits Byeong-soo, his Alzheimer's disease is at its worst, and he thinks he is a kid and Eun-hee is his dead sister.In the final scene, Byeong-soo is released from prison, walking on a desolate railroad. When he looks at Tae-joo's picture which is still on his daughter's locket, his perceptions take a turn for the worst: his Alzheimer's disease kicks in again, and he records, "Never trust your memory. Tae-joo is still alive," because many of his memories were unreliable before, he is suspicious of the memory of killing Tae-joo.Alternate Version: In the final scene, Byeong-soo is released from prison, walking on a desolate railroad. He switches his shoe, which he was wearing on the wrong foot. It's shown that he is the one who committed the Maiden Murders. Also he killed his police friend, who was planning to arrest him with the help of Eun-hee and Tae-joo. Then he sedates Eun-hee and kills Tae-joo and dumps his body along with the Silver Sonata in the reservoir. The movie fades out with Byeong-soo saying "Never trust your memory".