A teenage girl, Su-mi (Im Soo-jung), is being treated for shock and psychosis in a mental institution. She is released and returns home to her family's secluded estate in the countryside with her father (Kim Kap-soo) and younger sister Su-yeon (Moon Geun-young), whom she is protective over. The sisters have a cold reunion with their stepmother, Eun-joo (Yum Jung-ah), who constantly requires medication. Su-mi has a nightmare of her late mother's ghost. The next day, she finds family photos which reveal that Eun-joo was formerly an in-home nurse for her then-terminally ill mother. She discovers bruises on her sister's arms and suspects Eun-joo being responsible. Su-mi confronts Eun-joo about the bruises but Eun-joo refuses to apologize for her actions. That night, their uncle and aunt arrive for dinner, and Eun-joo tells bizarre stories that bewilder them. The aunt suddenly suffers a violent seizure and suffocates. After recovering, she tells her husband that she saw the ghost of a dead girl beneath the kitchen sink during her seizure. Eun-joo tries to see what is beneath the sink, but the ghost girl violently grabs her arm. Eun-joo's relationship with her stepdaughters sours after she finds her pet bird mutilated and killed and her personal photographs defaced. As punishment, she locks Su-yeon in the closet. Su-mi releases her and tells her father about the abuse. Her father begs her to stop acting out and informs her that Su-yeon is dead. Su-mi refuses to believe it as she sees her sister sobbing uncontrollably. The next morning, Eun-joo drags a bloodied sack through the house, whipping it. Su-mi believes that Su-yeon is inside the sack. Eun-joo and Su-mi get into a violent physical altercation. Su-mi's father arrives to find an unconscious Su-mi. It is ultimately revealed that Su-mi and her father were alone in the house the entire time. Su-yeon and Eun-joo were merely hallucinatory manifestations of Su-mi's dissociative identity disorder. Throughout the film, Su-mi simultaneously switched personalities, acting as herself and Eun-joo. She hallucinated Su-yeon as a result of not being able to accept her death. In her "Eun-joo" personality, Su-mi tries to sleep with her father as his "wife" and pretends to abuse Su-yeon by injuring herself and putting a porcelain doll in a sack and whipping it. It's also revealed that Su-mi was the one responsible for killing the pet bird. The father and the real Eun-joo, a somewhat different woman from the imaginary version, send Su-mi back to the mental institution. Eun-joo tries to reconcile with Su-mi, promising to visit her as often as she can, but Su-mi rebuffs her. That night, Eun-joo hears footsteps in Su-yeon's old bedroom, revealing that the ghosts actually existed. Su-yeon's real ghost crawls out of the closet and kills Eun-joo. Meanwhile, Su-mi smiles, appearing to have finally found peace. Flashbacks reveal the day that led Su-mi to be institutionalized. Whilst her terminally ill mother was still alive, her father engaged in an adulterous affair with Eun-joo, whilst she was still an in-home nurse. This upsets the sisters and drives their mother to hang herself in the closet of Su-yeon's bedroom. Su-yeon attempts to revive her but the closet collapses on top of her, slowly crushing her to death. Eun-joo walks in and considers saving Su-yeon, but she encounters Su-mi, who engages in a heated confrontation with her. Angry at Su-mi's insults, Eun-joo decides to leave Su-yeon to die and tells Su-mi that she'll "regret this moment." Su-mi leaves the house, unaware of both her sister and her mother's fates. !!!