The film opens inside a sterile psychiatric hospital, where a doctor gently interviews a catatonic teenage girl, Bae Su-mi. The narrative shifts to Su-mi and her timid younger sister Su-yeon arriving by car with their distant father, Moo-hyeon, at their secluded lakeside estate following their discharge from the clinic. The girls are greeted by their high-strung, hostile stepmother, Eun-joo, whom both sisters openly despise. Su-mi acts as Su-yeon's fierce protector, defending her against Eun-joo's constant scoldings.

Supernatural phenomena begin immediately inside the dark corridors. Su-yeon is plagued by severe nightmares of a bloody, disjointed woman climbing onto her bed. Su-mi experiences waking dreams of footsteps on the staircase and finds bruises on Su-yeon's arms, suspecting Eun-joo is physically abusing her sister. During an awkward family dinner with Eun-joo's brother and his wife, the sister-in-law experiences a violent seizure, claiming she saw a dead girl crouching under the kitchen sink.

Tensions boil over when Eun-joo's pet bird is found strangled to death inside its cage. Eun-joo blames Su-yeon, locks her inside a dark bedroom wardrobe, and leaves her to weep in panic. Su-mi breaks her sister out and confronts her father, begging him to intervene and stop Eun-joo's abuse. Moo-hyeon snaps in frustration, delivering a shattering piece of reality: Su-yeon is dead, and Su-mi has been imagining her presence all along.

In the house, Eun-joo beats a heavy, blood-soaked canvas sack with a stick and drags it across the floor. Horrified that Su-yeon is stuffed inside, Su-mi attacks Eun-joo in the hallway. A violent struggle ensues, and Eun-joo knocks Su-mi unconscious with a stone statuette. Moo-hyeon returns home to find Su-mi lying bloodied on the floor completely alone, revealing the film's shocking core truth: Eun-joo was never at the house.

Suffering from severe Dissociative Identity Disorder caused by intense trauma, Su-mi had been living alone in the house with her father, subconsciously alternating between acting as herself, hallucinating her deceased sister Su-yeon, and fully taking on the cruel persona of her stepmother Eun-joo. The bruises, the murdered bird, and the bloody sack were all manifested and carried out by Su-mi herself in her fractured states.

Moo-hyeon arranges for Su-mi to be recommitted to the psychiatric hospital. The real Eun-joo visits Su-mi in the asylum to offer a cold, polite farewell before returning alone to the empty country estate to pack her belongings. While standing in Su-yeon's old bedroom, Eun-joo hears heavy, rhythmic thuds coming from inside the dark wooden wardrobe. She opens the closet doors, only for a real, mangled female ghost—the genuine vengeful spirit of Su-yeon—to crawl out and brutally kill Eun-joo, exacting supernatural retribution.

A heartbreaking final flashback reveals the tragedy that destroyed the family. Months earlier, Su-mi and Su-yeon's terminally ill biological mother hung herself inside the bedroom wardrobe after discovering her husband was having an affair with their nurse, Eun-joo. Su-yeon entered the room and discovered her mother's hanging corpse. In a frantic attempt to pull her mother down, Su-yeon accidentally pulled the heavy wooden wardrobe over onto herself, crushing her beneath its weight.

Eun-joo heard the crash, investigated the room, and saw Su-yeon dying beneath the furniture. As Eun-joo prepared to help, a furious Su-mi stormed into the hallway, insulted Eun-joo, and marched out of the house in anger without realizing what was happening. In petty spite, Eun-joo chose to walk away and leave Su-yeon to suffocate to death beneath the wardrobe, telling Su-mi she would regret walking out. The movie ends on Su-mi sitting alone in the psychiatric ward, weeping as she remembers the sunny afternoon by the pier before guilt fractured her mind forever.