A teenage girl, Su-mi, 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 and younger sister Su-yeon, whom she is protective of. The sisters have a cold reunion with their stepmother, Eun-joo, who constantly requires medication. Eun-joo also has a strained relationship with her husband, both of them enduring a sexless marriage.

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 is 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-mi had suffered from dissociative identity disorder, where she possessed two personalities: herself and Eun-joo. The "body" in the sack that Su-mi was whipping was actually a porcelain doll and she was also the one who killed the pet bird. Su-yeon was also revealed to be long dead; her presence was actually the result of Su-mi's hallucinations.

The father and the real Eun-joo 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 ghost actually exists. 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. While her terminally ill mother was still alive, her father engaged in an adulterous affair with Eun-joo, when she was still their 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. Eun-joo sees Su-yeon thrashing and suffocating and at the last minute, decides to save Su-yeon. However, Su-mi arrives and criticises Eun-joo for interfering with her family. Angry at Su-mi's criticisms, Eun-joo leaves 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.

https://en.wikipedia.org/wiki/A_Tale_of_Two_Sisters
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