Memoir of a Murderer (Hangul: 살인자의 기억법; lit: A Murderer's Guide to Memorization) is a 2017 South Korean action thriller film directed by Won Shin-yun. It is based on a bestselling novel by Kim Young-ha. The film stars Sol Kyung-gu and Kim Nam-gil in the lead roles, with Kim Seol-hyun and Oh Dal-su in the supporting roles. Filming began in late 2015. The film released in South Korean theatres on 6 September 2017.
Plot
After killing his abusive father as a teenager, Byeong-soo (Sol Kyung-gu) begins to believe there are people who deserve to die. During his killing spree, he buries his victims in a bamboo grove. After killing a woman, he gets into a car accident and injures his head, resulting in degenerative Alzheimer's disease. He stops killing, resumes his job as a veterinarian, and continues caring for his daughter Eun-hee (Kim Seol-hyun).
After multiple visits to a nearby police station (as he forgets where he lives), Byeong-soo strikes up a friendship with policeman Byeong-man (Oh Dal-su). At Eun-hee's behest, Byeong-soo uses a recorder to maintain his routines. He types everything he can remember into a memoir and saves it on his laptop. He also goes to a poetry class, where his poetic depictions of murders attracts classmate Jo Yeon-joo (Hwang Seok-jeong), much to his annoyance.
When the small town is shaken by the murders of two young women, Byeong-soo is confused and wonders if he did the killing. On the way back from the bamboo grove (to check for new burial sites), Byeong-soo hits another car. He notices a bloody bag in the popped-open trunk and collects blood on his handkerchief. The car's driver claims it is a deer's corpse and drives away without exchanging contact information.
After ascertaining the blood sample is human, Byeong-soo anonymously calls the police, describing the encounter and the car's make and license plate. The tip is considered a prank as the car's driver is policeman Min Tae-joo (Kim Nam-gil). Byeong-soo also asks Byeong-man to run the license plate separately. Instinctively realizing Tae-joo is a serial killer, Byeong-soo deduces where he was taking the body to. He finds the body and tips the police, catching Tae-joo's attention.
Tae-joo goes to Byeong-soo's clinic, where he meets Eun-hee. After a short while, the two begin dating. Byeong-soo sees the pair but does not recognize Tae-joo. After Byeong-man tells him about the license plate's owner, Byeong-soo realizes Tae-joo is the killer who uses his daughter to taunt and threaten him. He gives Byeong-man the handkerchief with the blood, telling him it's from Tae-joo's car.
Suppressing his self-doubts, Byeong-soo is determined to kill Tae-joo, but his failing mind and body betray him. He starts experiencing hallucinations and a week's worth of memory is erased.
Byeong-soo wakes up to find himself tied up in his bedroom, with Tae-joo reading and liberally editing his memoir. Tae-joo admits to being the killer and having swapped the handkerchief with the blood sample. Byeong-soo tries to grab his bag for a weapon but fails and spills its contents onto the floor. Tae-joo threatens to kill Eun-hee unless Byeong-soo takes the blame for his crimes, then tranquilizes him. Byeong-soo wakes up and only vaguely remembers what happened.
Fearing for his daughter's safety, Byeong-soo orders her to stay with his sister Maria (Gil Hae-yeon) at her convent. He follows Tae-joo to an abandoned house and finds footage of Yeon-joo being held hostage. When he presents this evidence to the police, they suspect Byeong-soo himself is responsible. Upon trying to call Maria to prove Eun-hee's safety, Byeong-soo recalls Maria committed suicide shortly after their father's death. Police begin digging up the bamboo grove, finding the remains of past victims and Yeon-joo.
At home, Byeong-soo recalls who his final victim was - his adulterous wife. Before dying, she said Eun-hee was not his biological daughter. Arriving home after the car crash, Byeong-soo's injury causes him to forget this revelation and he continues caring for her. Byeong-soo remembers almost strangling Eun-hee, mistaking her for his wife, and becomes convinced that he might have killed his daughter in a fit of insanity.
Guilt-ridden, Byeong-soo prepares to kill himself. However, he then hears Tae-joo confessing his crimes as during their earlier confrontation, he accidentally turned on his recorder.