Nina Sayers is a 28-year-old dancer in a New York City ballet company, which is preparing to open its new season with Tchaikovsky’s Swan Lake. With prima ballerina Beth MacIntyre being forced into retirement, artistic director Thomas Leroy announces he is looking for a new dancer to portray the dual role of the innocent White Swan and the sensual Black Swan. Nina auditions for the role and gives a flawless performance as the White Swan, but fails to embody the Black Swan.
The following day, Nina asks Thomas to reconsider choosing her to play the role. When he forcibly kisses her, she bites him before running out of his office. Later that day, Nina sees the cast list and discovers, much to her surprise and that of her overprotective mother Erica, she will be portraying the lead. At a gala celebrating the new season, an intoxicated Beth confronts Nina, accusing her of sleeping with Thomas to get the role. The following day, Nina discovers that Beth was hit by a car while walking in the street and Thomas believes she did it on purpose.
During rehearsals, Thomas tells Nina to observe new dancer Lily, whom he describes as possessing an uninhibited quality that Nina lacks. Nina also falls victim to several hallucinations of a doppelgänger following her wherever she goes and finds unexplained scratch marks on her back. One night, Nina accepts Lily’s invitation to dinner despite Erica’s objections.
Over dinner, Lily offers Nina an ecstasy capsule to help her relax. Nina turns it down, but later accepts a drink laced with ecstasy powder. The two dance at a nightclub and return to Nina’s apartment late. After fighting with her mother, Nina barricades herself in her room and has sex with Lily. The following morning, Nina wakes up alone and realizes she is late for the dress rehearsal. Upon arriving at Lincoln Center, she finds Lily dancing as the Black Swan and confronts her about their night together. When Lily denies that the sexual part of the evening had ever taken place, Nina realizes that the memory was a hallucination.
After learning that Thomas has made Lily her alternate, Nina’s hallucinations grow increasingly strong to the point where her mother tries to prevent her from performing on opening night. Nina forces her way past her mother, shouting, “I’m the Swan Queen, you’re the one who never left the corps!”. She arrives at Lincoln Center only to discover that Lily is indeed set to take over her role. She confronts Thomas, who becomes so impressed by her confidence that he allows her to perform. During the end of the ballet’s second act, Nina becomes distracted by another hallucination, causing the Prince to drop her on stage. She returns to her dressing room and finds Lily preparing to play the Black Swan. When Lily transforms into Nina’s evil twin, the two engage in a fight that ends with Nina stabbing the doppelgänger with a shard of glass, only to reveal that it is actually Lily who has been stabbed. She hides the corpse and returns to the stage, where she loses herself and gives a flawless performance as the Black Swan.
Nina receives a standing ovation from the audience and, after surprising Thomas with a passionate kiss, returns to her dressing room. While changing, Nina hears a knock at the door and opens it to find Lily alive and congratulating her. Realizing the fight never occurred and that she had stabbed herself, Nina quietly continues changing and pulls a shard of glass from her own abdomen.
After dancing the final act, in which the White Swan attempts to commit suicide by throwing herself off a cliff but instead falls onto a hidden mattress, the theater erupts in thunderous applause while Thomas, Lily, and the rest of the cast all gather to congratulate Nina. They then discover that she is bleeding profusely. Thomas orders some of the dancers to go get help and frantically asks Nina what happened to her. Nina replies to him that her performance was perfect and loses consciousness as the screen slowly fades to blinding white light and the credits roll.