Nina Sayers (Portman), a young dancer with a prestigious New York City ballet company, lives with her mother, Erica (Hershey), a former dancer. The company is preparing to open the season with Swan Lake. The director, Thomas Leroy (Cassel), has to cast a new principal dancer after forcing Beth Macintyre (Winona Ryder) into retirement. Leroy wants the same ballerina to portray both the innocent, fragile White Swan and her dark, sensual twin, the Black Swan. Nina competes for the part. Although her audition goes badly, she asks Thomas to reconsider. He tells her she is ideal for the White Swan but lacks the passion necessary for the Black Swan. When he forcibly kisses her, she shows some spirit and bites him, and lands the part.
An intoxicated Beth angrily confronts Thomas and Nina. She is later hit by a car and seriously injured in what Thomas suspects was a suicide attempt.
Nina begins to witness strange happenings. Thomas, meanwhile, becomes increasingly critical of her "frigid" dancing and advises her to stop being a perfectionist and lose herself in the role. Thomas points to Lily (Kunis), another dancer in the company, whom he describes as lacking Nina's flawless technique but possessing an uninhibited quality that Nina has not shown.
The relationship between the two dancers is cool because of Lily's indiscretions, but Lily invites Nina to a night out. Nina is hesitant at first but decides to go against her mother's wishes. At a nightclub, Lily offers Nina a capsule to help her loosen up. Though reassured its effects will only last a few hours, Nina turns it down. Lily later slips it into her drink while she is absent. Nina returns home late, fights 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 that morning. After Lily tells her she spent the night with a man whom she met at the club, Nina realizes she imagined the encounter.
Nina's hallucinations become stronger as she sees Thomas and Lily have sex in a backstage area and Beth stabbing herself in the face at the hospital with a nail filer which Nina drops bloodied in the elevator. She has a violent argument with her mother, after which Nina passes out. Concerned about Nina's erratic behavior, her mother tries unsuccessfully to prevent her from performing on opening night; enraged, Nina stands up to her mother and forces her way out of the apartment. Since her mother had called to say Nina was sick, Thomas assigned understudy Lily to take over, but reluctantly gives way when Nina insists on performing.
The first act goes well, until Nina is distracted by a hallucination during a lift, causing her partner, playing the Prince, to drop her. Distraught, she returns to her dressing room and finds Lily there.As Lily announces she is to play the Black Swan, she transforms into Nina's double. Nina shoves her into a mirror, shattering it. She grabs a shard of glass and stabs her rival in the stomach, killing her. The corpse transforms back into Lily. Nina hides the body and returns to the stage to dance with passion and sensuality. Sprouting 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. Offstage, Thomas and the rest of the cast congratulate her on her stunning performance. Nina takes Thomas by surprise and kisses him.
Back in her dressing room before the final act, Nina is congratulated by Lily, showing that their fight was imaginary. The mirror, however, is still shattered. She removes a shard from her own body and realizes she had stabbed herself. Dancing the last scene, in which the White Swan throws herself off a cliff, Nina spots her mother weeping in the audience. As Nina falls backward onto a hidden mattress, the theater erupts in thunderous applause. Thomas and the rest of the cast gather to congratulate her – only to find that she is bleeding. She whispers, "I felt it. Perfect. It was perfect."