Carol Ledoux, a beautiful Belgian manicurist, lives in London with her older sister Helen. Carol is remarkably detached and struggles in her daily interactions. A suitor, Colin, is enamored with her and makes fervent attempts to court her, but Carol seems uninterested. When home, she enjoys looking out the window to see a group of nuns outside of their church playing soccer. Carol is troubled by her sister’s relationship with a man named Michael, whom she seems to dislike. She is bothered by his habit of leaving his razor and toothbrush in her glass in the bathroom. At night she is unable to sleep, haunted by the sounds of her sisters lovemaking. They are also constantly pestered by their landlord to pay their rent. When Carol walks home from work, she is bothered by a crack in the sidewalk. Colin happens upon her and she struggles to respond when he talks to her. He drives her home and tries to kiss her several times but she pulls away, running upstairs and vigorously brushing her teeth before weeping. That night Helen questions Carol for throwing away Michael’s belongings. At the manicure salon, Carol becomes increasingly distant, barely talking to her coworkers and customers. So much so that her boss decides to send her home for the day. After Helen and Michael leave for Italy on holiday, Carol is left alone in the apartment. At home, Carol takes a rabbit out of the fridge for dinner. Instead of cooking it, she is distracted by a number of Michael’s possessions left around the apartment, including a used article of his clothing, which she smells and makes her vomit. After trying on one of her sister’s dresses, she sees a dark figure in the mirror. That night, she hears footsteps outside her bedroom.
Her isolation begins to take its toll on her. One morning she runs a bath and walks away, causing it to overflow. As she turns on a light, the wall cracks open. She locks herself in her room and again hears footsteps. This time, she hallucinates that a man breaks into her room and rapes her. She is awoken in the hallway by a phone call from Colin but she hangs up. Carol misses three days of work. As she is giving a manicure, she stabs her client in the finger and is sent home early. The uncooked rabbit’s head is in her purse. At the apartment, she looks at an old family photo and the wall behind the photograph shatters like a mirror. After continuously being ignored, Colin shows up at her apartment. She refuses to open the door so he breaks in. He declares his love for her, and she responds by clubbing him to death with a candlestick. She cleans the blood, barricades the door, and places Colin’s corpse in the bathtub. In bed, she goes through the same rape hallucination. She wakes up the next morning, naked on the floor. Later, she walks down the dark hallway of her apartment where hands appear out of the walls and grab her. Later, the angry wife of Michael calls looking for Helen, causing Carol to cut the wire of the telephone.
The landlord comes looking for the rent. After he is unable to get in due to the barricade, he breaks into the apartment and sees Carol. She pays him the rent, but he is disgusted by the state of the apartment. He sees the uncooked rabbit, still sitting out, rotting. He propositions her, offering to forget about the rent, and makes an aggressive pass at her but she hacks him to death with Michael’s straight razor. She then sinks deeper into hallucination.
When Helen and Michael arrive home, Helen is dismayed at the state of the place. Michael happens on Helen hyperventilating and finds Colin’s dead body in the bath. Helen finds Carol under her bed in a catatonic state. Her neighbors flood in as Michael picks her up and carries her out, smiling. The final scene pans over items in the apartment, settling on a family photo showing Carol as a child, blankly staring down an older man we can assume is her father, while others in the photo smile for the camera.