Hae-won works in a Seoul bank. A stern, tense woman, her status and hyper competitive mentality make her apathetic to other people. After witnessing violence against a woman resulting in death, she refuses to identify the culprit because it's not her problem, even after the offender sexually harasses her. At work, she is accidentally locked in the toilet by the janitor and assumes one of her co-workers pranked her. She slaps her co-worker before realizing she is wrong. Hae-won is fired and accepts a friend's offer to take a vacation in Mudo, the desolate and socially backward Southern island where she spent her childhood.
On the island, Hae-won is warmly welcomed by Bok-nam, with whom she was close when they were teenagers, but whose constant letters she's since ignored. Life on the undeveloped island is hard, and Bok-nam is treated as little more than a slave by her abusive husband Man-jong, his lustful brother, and the local women. Bok-nam's love is reserved for her young daughter Yeon-hee, with whom she tries to escape for a better life. Man-jong is abusing and grooming Yeon-hee, to Bok-nam's rage. Yeon-hee is accidentally killed while trying to defend her mother against Man-Jong. Bok-nam, crying, reveals that Yeon-Hee may not be Man-Jong's daughter, as Bok-nam was raped by many local men.
In a flashback, Hae-won was teaching Bok-nam how to play a tune on her recorder when four local boys began harassing them. Hae-won fled and witnessed the boys assaulting the unconscious Bok-nam.
The locals lie to police when questioned about the death of Yeon-hee, and Hae-won does not support Bok-nam. Bok-nam snaps and begins killing everyone she comes across, armed with a sickle. After murdering three old women, she chases her mother-in-law to the cliff but swings the sickle blade out of the handle due to her poor eyesight. The mother-in-law sees Man-Jong and his brother returning by boat and tries to swim to the boat for help. Due to poor eyesight, she miscalculates the distance to the water and dies falling on the rocks below. Bok-nam decapitates her brother-in-law. Man-jong tries to kill Bok-nam, snapping Hae-won out of her apathy. She threatens to call the police, and he says he can either kill Hae-won or make her his new wife. Bok-nam seduces him by licking his knife. As Man-jong lets down his guard, she bites his finger. Man-jong hits her and tries to reach for an axe to kill her but Bok-nam uses her mouth to grab the knife and fatally stab Man-Jong. As Man-Jong clings to his life, Bok-nam hacks him to dead. She smears his body with bean paste in mockery of his earlier remark about curing her bruises with bean paste. Terrified, Hae-won runs to the boat. Bok-nam catches up and tries to drown the oarsman for helping Man-jong. As Hae-won tries to turn the boat, the oarsman is caught in the propeller and torn to shreds. Hae-won escapes to the mainland.
Bok-nam travels to Seoul with the childhood recorder and tries to kill Hae-won, who is in police custody. Hae-won did witness Yeon-hee's murder, but lied to the investigator about being asleep. A policeman shoots Bok-nam and she kills him with a sledgehammer. Hae-won kills Bok-nam, who dies in her lap, with the broken recorder. All the murdered victims have been buried or cremated by Bok-nam, leaving the island devoid of human life.
Traumatized and guilt-ridden, Hae-won points out the culprits of the sexual assault she witnessed to the police. She takes Bok-nam's letters out of the trash and reads them, regretting not helping Bok-nam when she had the chance.[4][5][6][7]
