Movie horror comes in many forms: scary sex scenes, frightening monsters and mental terrors. But some of the most haunting horror stories aren't merely terrifying, they're achingly sad.

"Black Swan," starring Natalie Portman as a lovely ingénue losing her footing, combines all those strands into a terrific, profoundly disturbing tapestry of suspense and madness. As its protagonist transforms herself to embody the schizoid heroine of "Swan Lake," the film picks at our deepest anxieties -- injury, disfigurement, loss of a coveted job, loss of identity, loss of sanity. In most fright films, danger lurks in the shadows. Here it's grinning from a mirror.

Portman plays Nina, a dancer in a fictional New York City company who is completely consumed by ballet. Shy, virginal, still under the thumb of her domineering stage mother (Barbara Hershey), she is an accomplished but over-controlled dancer. Her artistic director, Thomas (played with great gusto by Vincent Cassel), urges her to loosen up. Unless she can express the wild, dark sensuality of the Swan Queen's alter ego, known as the Black Swan, she will never win the iconic part.

But Nina clings to her obsessive self-discipline. It's what keeps her from spinning out of balance. The deeper she delves into the role, the harder she strains to differentiate between the dance company's backstage treacheries and the sinister world of her own imagination. Each pirouette adds to her downward spiral.

From this unusual premise, director Darren Aronofsky ("The Wrestler") fashions an excellent, thoughtful work of art with the giddy urgency of a slasher movie. Using a handheld camera, Aronofsky shoots intense, intimate close-ups that hold the characters in a clinch. In tightly framed shots of Nina performing, we don't see her dancing so much as her absorption in it -- the concentration of a professional who has become almost selfless. As the camera moves acrobatically through the performance, we experience Nina's ecstatic abandon. And when her grip on reality loosens, Aronofsky's camera recoils in horror along with his heroine.

Portman's performance as the psychologically disintegrating dancer is beyond praise. Her worry, guilt and grief are so potent they're nearly unbearable. Nina's not all that articulate, which makes Portman's accomplishment all the more impressive. She has to communicate volumes through expression alone, and she carries it off brilliantly.

When the actress' gifts mesh with the director's, "Black Swan" is at its best. Aronofsky has plenty of tricks up his own sleeve, of course. At one point during a passionate sex scene, special-effects goosebumps subtly brush across Nina's thigh. It's a tiny thing, but perfectly suited to the story of a repressed young woman who fears she is being possessed by a bird. Details like that add a bright spot of unease to an already disquieting story.

The film teases our expectations before it sweeps our legs out from beneath us. Nina fancies herself a persecuted maiden, and we're encouraged to accept her self-assessment, but then it begins to slip away. The French-born Cassel promises to be a debonair, manipulative sex bandit using the dancers as his personal harem. By the time the film is over, we have an entirely different understanding of the impresario as a man whose driving lust is artistic, not carnal. The same goes for smoky-voiced Mila Kunis as a rival dancer who may be undermining Nina's career. With her insinuating manner, she appears to be a standard mean girl in toe shoes. As the film progresses, we wonder how many of her hostile acts occurred and how many are Nina's projections.

There's an emotional richness to the film that balances against its stark, monochromatic palette. Nina is as human as real people we know, even as she succumbs to the world's worst case of bird flu.