Played by Natalie Portman in a brutally smashing, bruising, wholly committed performance, the young dancer, Nina, looks more like a child than a woman, her flesh as undernourished as her mind. When she goes to bed at night, a nearby jewelry box tinkling “Swan Lake,” a crowd of stuffed animals watches over her, longtime companions that — as Nina and this dementedly entertaining film grow more unhinged — begin to look more like jailers than friends.

Crammed with twins — mirrors too many mirrors, lookalikes, rebeling reflections, doppelgängers — the story follows that of the “Swan Lake” ballet in broad, gradually warped strokes. It opens with the artistic director of a fictional New York ballet company, Thomas (Vincent Cassel), announcing that the new season will begin with a “visceral and real” version of that old favorite. To that end he dumps his prima ballerina, Beth (Winona Ryder), and picks Nina to dance the dual role of the swan queen (an enchanted woman in bird form) and her villainous black twin. But as the pressure builds, things fall apart, or Nina does. She stumbles out of a spin and begins scratching at her skin. One day she strips a piece from her finger as lightly as if she were peeling a banana.

That isn’t a knock. One of the pleasures of “Black Swan” is its lack of reverence toward the rarefied world of ballet, which to outsiders can look as lively as a crypt. Mr. Aronofsky makes this world (or his version of it) exciting partly by pulling back the velvet curtains and showing you the sacrifices and crushingly hard work that goes into creating beautiful dances. Nina doesn’t just pirouette prettily, she also cracks her damaged toes (the sound design picking up every crackle and crunch) and sticks her fingers down her throat to vomit up her food. Mostly, though, she trains hard, hammering her toe shoes into floor much as Jake La Motta pounded his fists into flesh. She’s a contender, but also a martyr to her art.

Portman commands the screen and through her own career traces the trajectory of Nina. She has been the precocious young actress, the preternaturally aware girl raised by an assassin (The Professional), the youthful next-door neighbor in love with a grown but arrestedly developed man (Beautiful Girls), the little lost stepdaughter of a driven cop (Heat). It seems as if she would forever play these kind of roles; never able to grow up. Graced (or cursed) with good genes, she has waited for time to catch up with her psychological depth, and now she has danced right up to the precipice and has taken that final leap.

The girl is gone, and Black Swan takes flight. With Portman shouldering the burden onscreen, Aronofsky has crafted a spellbinding blend of Kubrickian execution and sensual pop psychosis to rival David Fincher’s Fight Club. It is a thing of beauty and perfection, this epic and tragic dance.