The plot follows a mother who, after her intellectually disabled son is accused of the murder of a young girl, attempts to find the true killer to get her son freed. It received critical acclaim from critics, who praised Kim's performance, the direction and screenplay, and the film's uniqueness. An unnamed widow lives alone with her only son, selling medicinal herbs in a small town in southern South Korea while conducting unlicensed acupuncture treatments for the town's women on the side to erase bad memories.
Her son, Yoon Do-joon, is shy, but prone to attacking anyone who mocks his intellectual disability. She dotes on him and scolds him for hanging out with Jin-tae, a local thug. When Do-joon is nearly hit by a car, he and Jin-tae vandalize the car and attack the driver and passengers as revenge. Jin-tae blames Do-joon for the damage done to the car, and Do-joon is sued. His mother struggles with the burden of the debt. On his way home from a bar late at night, Do-joon sees a high school girl named Moon Ah-jung walking alone and follows her to an abandoned building.
The next morning, she is discovered dead on the rooftop, shocking the town and pressuring the incompetent police to find the killer. Do-joon is arrested for the murder due to circumstantial evidence placing him near the scene of the crime. His mother believes he is innocent, and tries to prove he is not the murderer. However, she is challenged by her self-absorbed lawyer and the community who unanimously blames Do-joon for the crime. Suspecting Jin-tae of committing the murder, the mother breaks into his house to look for evidence.
She takes a golf club, which she believes has blood on it, but when she turns it over to the police and Jin-tae is confronted about it, it becomes clear that the "blood" is just smeared lipstick. Despite her accusation, Jin-tae agrees to help the mother solve the case for a fee. When the mother questions the people in town about Ah-jung, they tell her the girl was sexually promiscuous and in a relationship with a boy known as Jong-pal, who had escaped a sanatorium.
Do-joon attacks another prisoner who calls him "retard". On one of his mother's prison visits, Do-joon recalls a memory of her attempt to kill him and then herself when he was five by lacing their drinks with a pesticide.
