The film begins with a girl named Su-Mi being brought to a psychiatrist by a nurse. The girl remains unresponsive despite the psychiatrist's questioning, until he shows her a picture of her and her family. Su-Mi is eventually brought home by her father (Moo-Hyeon) along with her timid sister, Su-Yeong, where they are met by their difficult stepmother (Eun-Joo).
That night Su-Yeong awakens from her sleep to find somebody entering her room and she flees to her sister's room in terror. Su-Mi promises Su-Yeong that she will always be there for her, and they fall asleep.
Su-Mi awakens after a nightmare to find that Su-Yeong has begun her period, as has her stepmother, on the very same day. The girls look at some old photos, which reveal that Eun-Joo was a doctor and a coworker of their father. Su-Mi sees bruises on Su-Yeong's arms and accuses Eun-Joo.
At dinner with guests—Sun-Kyu (Eun-Joo's brother) and his wife—Eun-Joo acts strangely. During dinner, Sun-Kyu's wife begins to choke and convulse on the floor.
Eun-Joo insists that strange events have become frequent since Su-Mi and Su-Yeong returned. Her suspicions are only intensified after her pet bird is slain and found in Su-Yeong's bed. She goes into a rage and locks Su-Yeong in the wardrobe, and after Su-Yeong is freed the father asks Su-Mi why she is making trouble. She insists that the stepmother harasses Su-Yeong, locking her in the closet, her father informs her that Su-Yeong is dead, and that she is not getting better. Su-Mi wakes to find her sister missing and believes she sees her stepmother dragging around a bloody bag through the corridors of the house, and beating it. Su-Mi runs to the kitchen to get a knife to free her sister, but Eun-Joo attempts to stop her by splashing boiling water from the kettle onto her. A fight ensues and Su-Mi is rendered unconscious and dragged away.
The father returns home, having left earlier, to find an injured Su-Mi, however all evidence is suspiciously gone as are the stepmother and bag. The scene shifts, Su-Mi is now the stepmother. She looks up in surprise, and the camera rotates to show the real stepmother, standing in the door way, looking sympathetic. When the camera turns around again, Su-Mi is back where she was. She takes the pills.
It is revealed, that all along Su-Yeong really has been dead, and Su-Mi was acting as the stepmother and sister because of a severe multiple personality disorder she suffers from and thus she was the guest at dinner who suffered a convulsion, was hugging herself that night, had the nightmare and was beating the bag (which had a bloodied doll in it).
Sun Mi than flashes back to a time when Eun-Joo was a live-in nurse caring for the girls' sick mother. The girls sense that Eun-Joo and their father are flirting. Su-Mi displays her disapproval toward Eun-Joo, who in-turn takes her anger out on Su-Yeong. Su-Yeong then returns to her room and collapses onto her bed, in tears. The girl's mother is shown trying to comfort her as she too starts crying.
The flashback ends, and the stepmother sits alone in the quiet home. A noise comes from Su-Yeong's room. When she investigates, the wardrobe opens and from it crawls out the ghost, presumably of either Su-Yeong or her mother. The scene ends ambiguously with a scream.
The flashback picks up again from when Su-Yeong had fallen asleep, with her waking up and discovering her dead mother, who had died of an overdose, in her wardrobe. She shakes the body, trying to wake her mother up, and accidentally pulls the wardrobe down on top of herself.
Various people in the house hear the bang but only Eun-Joo (the nurse and future stepmother) goes to investigate. She discovers Su-Yeong screaming for help and attempting to claw at the wardrobe. Eun-Joo leaves rather than helping Su-Yeong, but then changes her mind and heads back for the scene. En route, she passes Su-Mi coming out of her room to investigate the noise. The sight of Eun-Joo angers Su-Mi. She forgets about the noise and starts disparaging Eun-Joo, who responds by not mentioning Su-Yeong's plight to Su-Mi, and tells the hostile Su-Mi that she will regret the moment. Su-mi angrily marches out of the house, passing her father who comes back to investigate the noise. The father assumes that the noise was Su-Mi's and does not enter the house.
The scene switches back to Su-Yeong crying out for Su-Mi's help. Outside the house Su-Mi turns back long enough to exchange angry stares with her stepmother on the balcony. The stepmother goes inside and Su-Mi turns once more from the house. The film ends as Su-Mi walks away from the house unaware of her dead mother and dying sister.