Nina Sayers (Natalie Portman), a young dancer with a prestigious New York City ballet company, lives with her mother, Erica (Barbara Hershey), a former dancer who ended her career at 28 when she became pregnant with Nina.
The ballet company is preparing for a production of Swan Lake. The director, Thomas Leroy (Vincent Cassel), has to cast a new principal dancer after forcing his present principal dancer, Beth Macintyre (Winona Ryder), into retirement. The lead must be able to portray both the innocent, fragile White Swan and her dark, sensual twin, the Black Swan. Nina is selected to compete for the part alongside several other dancers. After her audition goes badly, she asks Thomas to reconsider and give her the role. He tells her that her rigid technique makes her ideal for the White Swan, but she lacks the passion to dance the Black Swan. When he forces a kiss on her, she bites him. Later, she is chosen for the Swan Queen. An intoxicated Beth angrily confronts Thomas and Nina, and she is later hit by a car and seriously injured in what Thomas believes was a suicide attempt.
Nina begins to witness strange happenings around her. Thomas, meanwhile, becomes increasingly critical of Nina’s « frigid » dancing as the Black Swan and tells her she should stop being such a perfectionist and simply lose herself in the role. She makes the acquaintance of Lily (Mila Kunis), another dancer in the company whom Thomas described as having the qualities Nina lacks. The relationship between the two dancers cools because of Lily’s indiscretions, but to make up for it, Lily appears at Nina’s door and invites her for a night out. Nina is hesitant at first, but decides to join Lily against her mother’s wishes. While out, Lily offers Nina a capsule of Ecstasy. Upon returning to the apartment, Nina has another fight with her mother, barricades herself in her room and has sex with Lily. Next morning, Nina wakes up alone and late for rehearsal. When she arrives at the studio, she finds Lily dancing as the Swan Queen. Furious, she confronts Lily and asks her why she did not wake her up in the morning. After Lily tells her she spent the night with a man whom she met at the club, Nina realizes she imagined the whole encounter.
Nina’s hallucinations become stronger during rehearsals and at home, which culminates in a violent fight with her mother after which she passes out. Concerned about Nina’s erratic behavior, her mother tries to prevent her from attending the opening performance, but Nina forces her way through, insisting she can dance. Lily and Thomas are puzzled at her arrival since Nina’s mother had called saying she was sick.
The first act goes well until Nina is distracted by a hallucination during a lift, and the Prince drops her. Distraught, she returns to her dressing room and finds Lily dressed as the Black Swan. As Lily announces her intention to play the Black Swan, she transforms into Nina’s double. Nina struggles with her double, and shoves her into a mirror, shattering it. She grabs a shard of glass and stabs her double in the stomach, but the double has transformed back into Lily. She hides the body, returns to the stage, and dances the Black Swan passionately and sensually. Growing black feathers, her arms become black wings as she finally loses herself and is transformed into a black swan. At the end of the act, she receives a standing ovation from the audience. When she leaves the stage, she finds Thomas and the rest of the cast congratulating her on her stunning performance. Nina takes him by surprise and kisses him.
Back in her dressing room preparing for the final act, the dying of the White Swan, Nina is congratulated by Lily on her performance as the Black Swan. Nina suddenly realizes her fight with Lily, just as all the strange visions she had experienced, were hallucinations, but sees the mirror is still shattered. She notices a wound on her body and realizes that she stabbed herself, not Lily.
Back on stage, dancing in the last moments of the ballet when the White Swan throws herself off a cliff, she spots her mother weeping in the audience. As Nina falls, the theater erupts in thunderous applause, and Thomas and the rest of the cast congratulate her on her performance. Lily gasps in horror to see that Nina is bleeding. As Nina lies wounded, she stares up at the stage lights, whispering « I felt it – I’m Perfect – It was perfect », as the screen fades to white, the audience chanting her name.