A teenage girl, Su-mi, currently undergoing treatment for shock and psychosis in a mental institution, is eventually brought back to her family's secluded country estate by her father, Moo-hyeon, and her younger sister, Su-yeon, whom she feels a strong protective instinct for. Upon arrival, the sisters experience a chilly reunion with their stepmother, Eun-joo, who seems to be in constant need of medication due to her mental health struggles. The relationship between Eun-joo and Moo-hyeon is also notably tense, characterized by an emotional distance and the absence of intimacy in their marriage.
Throughout their time together, Su-mi and Eun-joo engage in numerous verbal confrontations. Moo-hyeon remains indifferent to Su-mi's grievances and dismisses her requests to remove a wardrobe in Su-yeon's room, believing it will only upset her. One troubling night, Su-yeon is plagued by a nightmare involving the ghost of their deceased mother and seeks comfort in Su-mi's room. As Su-yeon finds solace, Su-mi too experiences a haunting vision while sharing the bed with her sister. The following day, a shocking discovery leads Su-mi to find family photos that reveal Eun-joo was once the in-home nurse for her terminally ill mother. Alarmed, she observes bruises on Su-yeon's arms, suspecting Eun-joo's involvement. However, Su-yeon remains uncommunicative, only managing to express her distress through tears when confronted, which deepens Su-mi's growing anger towards Eun-joo, who coldly refers to the marks as "retribution."
The family dynamic takes a further turn during a dinner visit from their uncle, Sun-kyu, and aunt, Mi-hee. In an unsettling moment, Mi-hee suffers a seizure, and upon regaining her composure, claims to have seen "a girl" beneath the kitchen sink during her ordeal. This conviction prompts Eun-joo to inspect the area, where a hair clip mysteriously appears, only for a ghostly presence to violently grasp her arm when she reaches for it.
Eun-joo’s rapport with her stepdaughters deteriorates rapidly after her pet bird is discovered dead and private photos are vandalized. Suspected of the vandalism, Eun-joo retaliates by locking Su-yeon in the wardrobe. Su-mi manages to free her sister and pleads with Moo-hyeon after he uncovers another deceased bird. However, Moo-hyeon drops a bombshell revelation, asserts that Su-yeon is dead, leading Su-mi into an emotional spiral as she insists her sister is by her side. The tension escalates when Eun-joo is seen dragging a bloodied sack through the house, which Su-mi fears contains Su-yeon. This culminates in a brutal altercation between Eun-joo and Su-mi, ultimately ending with Su-mi losing consciousness.
As the story unfolds, it becomes clear that only Su-mi and her father actually inhabit the house. Struggling with dissociative identity disorder, Su-mi harbors two identities: her own and a harsher persona reminiscent of Eun-joo. The sack turns out to hold a porcelain doll, and the revelations deepen, confirming that Su-yeon has been dead and that her presence was an illusion crafted by Su-mi’s troubled mind. With Eun-joo seeking to mend their fractured relationship, she promises visits while Su-mi's rejection is marked by a painful gesture, mirroring the injuries seen on Su-yeon. That night, Eun-joo is left haunted by the sounds emanating from Su-yeon’s former room, where the lost girl’s spirit emerges from the wardrobe and ultimately brings about Eun-joo’s demise. In the meantime, Su-mi finds a bittersweet peace as she smiles through tears, suggesting a resolution to her turmoil.
In a poignant flashback, the events leading to Su-mi's institutionalization surface. A young Moo-hyeon engages in an affair with Eun-joo, causing deep rifts within the family while their terminally ill mother is still alive. The sisters are distraught, and after a confrontation during a tense family dinner, their mother’s tragic end serves as the catalyst for all ensuing events. The scene unfolds with Su-yeon discovering their mother’s lifeless body in the wardrobe, an accident that seals the fates of everyone involved when the wardrobe collapses upon them amidst the chaos, witnessed by all. Even as Su-mi chases after the truth, the losses they suffer weave a tragic tapestry that locks them in cycles of pain, regret, and unresolved conflicts.
