The film begins with Nina Sayers (Natalie Portman), a young ballerina in her twenties, performing the prologue to Swan Lake. This iconic ballet tells the story of a princess cursed to become the White Swan, who can only revert to her original form if a man pledges eternal loyalty to her. Her tragedy deepens as she faces betrayal from the Black Swan, a sorcerer's daughter, who is transformed to deceive the prince in love with the princess. Ultimately, the princess, consumed by despair over the prince's infidelity, takes her own life, resigning her fate to remain a swan forever. As Nina dances and envisions the curse being placed upon her character, she awakens in her apartment, realizing it was merely a dream. She tells her mother about the dream, who inattentively brushes her off. Nina shares that Thomas Leroy (Vincent Cassel), the director of her ballet company, has promised her a more significant role this season, to which her mother agrees, recognizing Nina's longevity in the company.
However, Nina's excitement turns to anxiety when she discovers that Beth (Winona Ryder), the aging lead dancer, is being sidelined due to her age. This shift signifies an opportunity, as Thomas seeks a fresh face for the lead in a revamped Swan Lake. He announces auditions, tapping some dancers on the shoulder for the principal studio, while others are to consider themselves dismissed. Witnessing Beth's emotional breakdown in her dressing room, Nina impulsively swipes various items from her before sneaking out.
During auditions, Nina showcases her flawless interpretation of the White Swan, but falters under pressure when asked to embody the Black Swan—only to be distracted by the arrival of Lily (Mila Kunis). Despite her expertise, Thomas critiques her performance, insisting she fails to convey the Black Swan's sensuality. Returning home defeated, Nina's emotional turmoil escalates as her mother downplays her feelings. The next day, Nina boldly requests the Black Swan role from Thomas but learns he has chosen another dancer, Veronica (Ksenia Soto). A moment of passion unexpectedly unfolds when Thomas kisses her, leaving Nina confused and distressed, which impresses Thomas rather than diminishing her value.
Following the announcement of her new role, Nina faces a maelstrom of conflicting emotions and visions, revealing a darker part of herself. Overwhelmed by the pressures of her performance, she encounters Lily in increasingly uncomfortable situations, adding layers of jealousy and competition to their relationship. Her frustration only deepens when she witnesses Beth's tragic accident, heightening her fear of aging and inadequacy in her craft.
As the story unfolds, Nina's psyche becomes increasingly unstable. She battles with her mother, who tries to safeguard her fragile mental state, but ultimately, Nina's fiercely competitive spirit drives her to an intense confrontation with Lily. The tensions culminate in an explosive fight, where Nina fatally stabs Lily—deluded into believing she’s still in control.
This harrowing act propels Nina into a remarkable performance as the Black Swan, where she fully realizes her transformation, earning the standing ovation she desperately needed. Yet, her triumph is bittersweet, marred by the bloodshed and turmoil she's unwittingly unleashed within herself. In her final act, she embodies the tragic resolution of the White Swan, surrendering to the very perfection she sought, uttering the fateful words, > "I was perfect," just moments before slipping into oblivion amidst the applause.
