A teenage girl, Su-mi, is being treated for
shock and
psychosis in a mental institution. She is eventually released and returns home to her family's secluded country estate with her father Moo-hyeon and younger sister Su-yeon. The sisters' stepmother, Eun-joo, constantly requires
medication. Su-mi and Eun-joo verbally clash multiple times. Moo-hyeon is uninterested in Su-mi's complaints and rejects her request to remove a wardrobe in Su-yeon's room, as it seems to upset Su-yeon. One night, Su-yeon has a nightmare of her late mother's
ghost and flees into Su-mi's room for comfort. Afterwards, Su-mi also has a nightmare with a ghost while Su-yeon sleeps next to her. The next day, Su-mi finds family photos which reveal that Eun-joo was formerly an in-home nurse for her then-
terminally ill mother. She discovers bruises on Su-yeon's arms and suspects Eun-joo to be responsible. Su-yeon neither confirms nor denies this. Instead, she begins sobbing and runs away when Su-mi gets angry about her silence. Su-mi confronts Eun-joo about the bruises, but Eun-joo refuses to apologize, explaining that they were "retribution." That night, the sisters' uncle, Sun-kyu, and aunt, Mi-hee, arrive for dinner, and Eun-joo tells bewildering stories involving Sun-kyu, which he denies remembering. Mi-hee suffers a violent
seizure and almost suffocates. After recovering on their way home, Mi-hee tells Sun-kyu that she saw "a girl" beneath the kitchen sink during her seizure. Eun-joo searches the kitchen when the cupboard below the sink opens by itself. She sees a hair clip on the ground, but the ghost girl violently grabs her arm when she tries to pick it up. Eun-joo later finds her pet birds killed and her personal photos defaced. She believes that either one or both of her stepdaughters are responsible and locks Su-yeon in the wardrobe. Su-mi releases her and tells Moo-hyeon, who had just found and buried another dead bird, about Eun-joo's abuse. Moo-hyeon begs her to stop acting out and exasperatedly reveals that Su-yeon is dead. Su-mi refuses to believe it as she is sure that her sister is right next to her, sobbing uncontrollably. The next morning, Eun-joo drags a bloodied sack through the house, whipping it in anger. Su-mi believes that Su-yeon is inside the sack. Eun-joo and Su-mi get into a physical altercation. Moo-hyeon arrives to find an unconscious Su-mi. It is ultimately revealed that Su-mi and her father were alone in the house all along. Su-mi is suffering from
dissociative identity disorder and is a system with two alters: herself and a ruder, more distant variation of her stepmother Eun-joo. The "body" in the sack that Su-mi was whipping was actually porcelain dolls, and she was also the one who killed the birds. Su-yeon is also revealed to be long dead, her presence having been a
hallucination by Su-Mi. Moo-hyeon and the real Eun-joo, a much different woman than what Su-mi had envisioned, 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 and forcefully grabs her arm, mirroring the injury Su-mi had discovered on Su-yeon days earlier. After they leave, Su-mi hears Su-yeon's ghost whistle their mother's favorite tune. That night, Eun-joo hears the same tune and footsteps in Su-yeon's old bedroom, revealing that the ghost actually exists. Luring her to her former bedroom, Su-yeon's ghost crawls out of the wardrobe and kills Eun-joo. Meanwhile, Su-mi lies back in her bed and smiles with a rolling tear, finally finding some kind of peace, knowing that Su-yeon is still with her in some form. A flashback reveals the day that led Su-mi to be institutionalized. While his terminally ill wife is still alive, Moo-hyeon engages in an
adulterous affair with Eun-joo, when she was still their in-home nurse. This upsets the sisters and, after comforting Su-yeon following a confrontation during a family dinner, drives their mother to kill herself in Su-yeon's bedroom wardrobe. Finding her mother dead, Su-yeon grabs her corpse in shock and begins to shake the wardrobe in a panic, trying to free her. The wardrobe collapses on top of them, which everyone in the house and outside hears. Eun-joo investigates and sees Su-yeon thrashing and suffocating. She runs out in a panic but hesitates and turns back around in the hallway. However, in that same moment Su-mi runs into her. Eun-joo asks her about the noise, but Su-mi ignores the question and mouths off at Eun-joo for intruding on their family's lives. Lashing out at each other, they have an argument while Su-yeon is shown to still be alive but in pain and slowly dying in her room. Angry, Eun-joo grabs Su-mi's arm and tells her that she "will regret this moment" and, unknown to Su-mi, leaves Su-yeon to die just to spite her. Su-mi storms out of the house, leaving Eun-joo visibly shaking in the hallway, while Su-yeon cries and struggles under the wardrobe. Su-mi walks away from the house in a quick, but decelerating stride, watched by Eun-joo from an upstairs window. Su-yeon dies, begging her sister to help her with her last breath. ==Cast==