The movie opens in a mental hospital where a teenage girl by the name of Su-Mi is suffering from shock and psychosis. She is being questioned by a doctor who shows her pictures of her family and asks if she can speak about the day that led her to being admitted to the hospital.
The next scene shows her returning with her father and sister, Su-Yeon, to the family's secluded estate near a lake. The two sisters are reluctant to go inside the house where their stepmother awaits. Instead they go down to a pier by the lake where they relax by putting their feet in the water and enjoy the sunshine.
Soon, they are called back to the house where they are greeted by the stepmother with a forced welcome. She chastises them for making her wait and taunts them by asking if they are feeling better after having been away. The two sisters are unresponsive and quickly leave to other parts of the house.
At a tense family dinner, the stepmother announces she has invited the sister's uncle and his wife to dinner the next day. Su-Mi tells her stepmother she will not eat with them and the two sisters leave the table. Su-Mi tells Su-Yeon that if their stepmother ever goes after Su-Yeon to let her know.
That night as Su-Yeon goes to sleep, she hears noises and the door to her bedroom creakily opens and a female hand sneaks around it. Su-Yeon pulls the covers over her head only to have them pulled back by an unseen force. Terrified, she runs out of her room to Su-Mi and tells her that someone has come into her room.
When Su-Mi goes to investigate, she only finds her father sleeping, but her stepmom catches her and tells her not to wake him. She rejoins Su-Yeon and comforts her, telling her that she will always be with her and they fall asleep.
Later, Su-Mi has a nightmarish dream about blood and a hand desperately slapping at the floor. She wakens to see something moving in her room. Crawling along the floor is the apparition of a woman whose face is covered by long black hair. The wraith becomes aware that Su-Mi is looking at her and she none too slowly floats over the top of her. The ghost is hanging in mid-air with blood running down her leg. A hand suddenly appears from under her dress. Su-Mi awakes with a start realizing everything she has witnessed was still a dream.
Tension continues to rise among the three females as their menstrual cycles become synched together.
Su-Mi finds several family photos which reveal that their stepmother was once a nurse her father worked with and apparently was also a live-in nurse for the girl's mother. Su-Yeon enters the room and Su-Mi discovers bruises on her arm, but when she angrily questions her if their stepmother was the cause, Su-Yeon refuses to answer and runs out. Su-Mi confronts her stepmother about Su-Yeons injuries. The stepmother angrily responds to Su-Mi by telling her that she is now the sister's mother and that they must accept it and bear the worst that life has to offer. She also questions if Su-Mi is feeling well.
That night, the sister's uncle and his wife arrive for a visit. They dine with the father and stepmother, whose behavior is extremely exaggerated and odd. When she asks the uncle if he remembers a peculiar event he tells her he doesn't. She responds icily: "Why don't you remember? Are you crazy?"
Suddenly, the uncle's wife starts to convulse. She is thrown to the floor and has a violent attack. When she recovers the uncle drives her home and she informs him that during her attack she saw a girl under the kitchen sink. The stepmother sees the girl too and tells the sister's father that ever since Su-Mi and Su-Yeon returned home weird things have been happening inside the house. The father seems unconvinced.
The stepmother goes into Su-Yeon's room where she discovers that photos of her have been mutilated and her face has been scratched out. Enraged, she attacks Su-Yeon and throws her into a wardrobe closet and locks her inside and refuses to let the terrified girl out. When the stepmother leaves the room, Su-Mi suspecting something is wrong, enters the bedroom and releases her sister from the closet. Overcome, she comforts Su-Yeon and tells her that she is sorry, that she didnt' hear Su-Yeon's pleas for help and that this will never happen again.
The father enters and asks Su-Mi why all these bad things have been happening since she returned. Su-Mi says that it's the stepmother to blame and that his new wife has been attacking Su-Yeon. The father tells her to stop it and screams at her that Su-Yeon is dead. Neither Su-Mi, nor Su-Yeon, is pleased to hear this news.
The father makes a phone call and informs the other person that he/she must come to the house tomorrow. The next morning, the stepmother is shown dragging a large sack with something inside it into the house and she starts beating it with a stick. The noise awakens Su-Mi. When she gets up she finds a note from her father saying he will return to the house that afternoon.
Su-Mi runs to find Su-Yeon and sees a trail of blood on the floor. Following it leads to the sack the stepmother dragged in earlier. Su-Mi thinks it is Su-Yeon inside, but she can't get the bag open. She goes to the kitchen to retrieve a knife and when she returns she finds that the sack has been moved to the wardrobe closet. She also finds that her stepmother is behind her ready to throw boiling water on her. A violent fight ensues between the two females and Su-Mi stumbles and is knocked unconscious. Her stepmother drags her to another part of the house and retrieves a garden statue.
As Su-Mi regains consciousness, the stepmother asks her what brought them to this point and reminds Su-Mi of a conversation the two once had where the stepmother told her that one day Su-Mi would live to regret something and that no matter how hard Su-Mi would try to forget it, she would never be able to. The stepmother then proceeds to drop the statue on Su-Mi's head as the father returns.
He finds an unconscious Su-Mi as well as the injured stepmother. He attends to the stepmother's injuries and leaves the room. When the door opens again, someone else enters: THE REAL STEPMOTHER. How can this new person be the stepmother when the orignial stepmother is sitting right there looking at herself?
The camera turns 360 degrees to reveal that Su-Mi has been in the house alone with her father the past few days and that the conversations and events she had previously had with Su-Yeon and her stepmother were the result of her schizophrenic mind.
The father and real stepmother take Su-Mi back to the hospital and the stepmother returns to the house alone that evening. There she hears an odd noise upstairs in what was once Su-Yeon's bedroom. When she goes to investigate she encounters a ghost hiding in the wardrobe closet.
Flashback scenes reveal that one day some time ago, the father visited the house with the stepmother after what was an apparent marriage, or engagement, to his mistress. His ex-wife, the uncle and the uncle's wife are also there. An argument results and the sister's real mother goes into Su-Yeon's room. When her daughter goes to sleep the mother hangs herself in Su-Yeon's wardrobe closet. The apparition Su-Mi dreamt of earlier was her hanged mother.
In the reveal, Su-Yeon wakens and the door to her closet opens. When she looks inside she finds her mother hanged. She shakes the lifeless body in an attempt to revive it, but instead pulls the wardrobe closet down on top of her. There she suffocates, or is slowly crushed to death, by the weight of her mother and the closet. She desperately strikes at the floor with her hand and calls out for Su-Mi to help her.
We also learn that the stepmother having heard something upstairs goes into Su-Yeon's room. There she discovers the turned over wardrobe, but apparently thinks it is the father's wife under the closet. She does nothing to help her and quickly leaves the room. She encounters Su-Mi and the two exchange words. The stepmother tells Su-Mi that Su-Mi will one day regret something and that she will not be able to make it go away. Su-Mi brushes past her and leaves the house. She hesitates and thinks about going back inside, but instead she goes for a walk down a pathway not knowing that her life has changed forever and that she won't be able to ever make the past events go away.