Nina Sayers, a young dancer with the company of New York City Ballet, lives with her overprotective mother, Erica, herself a former ballerina. The company is opening the season with Tchaikovsky's Swan Lake. After forcing prima ballerina Elizabeth "Beth" MacIntyre into retirement, artistic director Thomas Leroy announces he is looking for a new dancer for the dual roles of the innocent, fragile White Swan Odette and the sensual, dark Black Swan Odile. Nina auditions for the roles and gives a flawless dance as Odette, but fails to embody Odile.
The next day, Nina asks Thomas to reconsider but when he forcibly kisses her, she bites him and runs out of his office. Later that day, Nina sees the cast list and learns to her surprise she has received the lead role. At a gala celebrating the new season, an intoxicated Beth accuses Nina of providing sexual favors to Thomas in return for a promotion. The next day, Nina hears Beth was hit by a car; Thomas believes she was attempting suicide. Nina visits an unconscious Beth in the hospital and is distraught to see her critically injured legs, meaning Beth will no longer be able to perform as a dancer.
During rehearsals, Thomas tells Nina to observe a newcomer, Lily, who has a physical resemblance to Nina but also an uninhibited quality Nina lacks. Nina has hallucinations and finds scratch marks on her back. One night, despite Erica's objection, Nina accepts Lily's invitation to go out for drinks. Lily offers Nina an ecstasy capsule, saying it would help her relax. Nina turns it down at first but then accepts. She repeats Lily's assurance that the effects will only last for a few hours, and quickly begins to act under the ecstasy's influence. Nina flirts with men at the bar and Lily as well. After the two dance at a nightclub, they go back to the apartment, where Lily seduces Nina. The next morning, she wakes up and realizes that she is late for the dress rehearsal.
Arriving at Lincoln Center, Nina sees Lily dancing as Odile and confronts her about their night together. Lily seems confused by Nina's insinuation that they slept together and denies going home with Nina, saying she went home with one of the men from the bar. Nina becomes convinced Lily intends to take her place, especially after learning that Thomas has made Lily her alternate. Nina's hallucinations grow stronger and her injuries increase, going as far as hallucinating herself transforming into Odile. On opening night, Nina berates her mother for calling the theatre and telling them she was not well enough to perform, worried that the role might be too much for her. When Nina arrives late, Lily is prepared to replace her, but Nina convinces Thomas to allow her to take back her role.
Towards the end of the ballet's second act, Nina is distracted by another hallucination and loses her stability as Odette. This causes the male dancer playing the prince to drop her on stage, which infuriates Thomas. She returns to her dressing room and finds Lily preparing as Odile. During a confrontation, Lily transforms into Nina. The two fight, breaking a mirror. Nina stabs her doppelgänger with a large shard of glass from the mirror, killing her. The body reverts to Lily. Nina hides the corpse in the bathroom and takes the stage, dancing flawlessly as Odile and seemingly turns into a black swan, her arms covered in feathers. Amidst a standing ovation from the audience, Nina surprises Thomas with a passionate kiss and returns to her dressing room.
As Nina resumes the Odette tutu and white swan makeup, she hears a knock at her door. She opens it to find Lily alive, who apologizes for the misunderstanding and congratulates Nina before taking her leave. Confused, Nina sees the mirror is still broken, and the towel she used to mop up the blood is clean with no corpse in the bathroom. She looks down and pulls a piece of glass from her abdomen, realizing she had stabbed herself instead. Nina dances the final act of the ballet, which ends with Odette throwing herself off a cliff and Nina landing on a mattress. The theatre erupts in thunderous applause while Thomas, Lily, and the others gather to congratulate Nina, who remains lying on the mattress. Thomas sees the blood spreading at her waist and shouts for help. He frantically asks Nina what happened to her, to which she calmly replies: "I felt it. It was perfect" as the screen fades to white.
