WHILE Fish Tank is a film about the kind of people who name their dog after a brand of lager, there isn't an ounce of patronization in it. Director Andrea Arnold's film is the furthest thing from a slumming expedition, although the material is lurid: the illegal romance between an adult and a 15-year-old girl named Mia (Katie Jarvis) living in a public housing project on the east coast of England.

Mia is a small, thin, feral girl at odds with everyone. She's a dropout and a delinquent, and the special-ed school gates are gaping wide for her. At home, her lounging mother, Joanne (Kierston Wareing), barely tolerates her; Tyler (Rebecca Griffiths), her little sister, isn't a fan either. Mia is always ready to have it out with the pack of teenage girls who roam the housing project's corridor. Mia's therapy for the stress is dancing: practicing until she's breathless in her hideout, the living room of an abandoned flat in this concrete tower.

Her mother has just made a real find: Connor (Michael Fassbender), a man with a job. We know this because Mia discovers a pay stub when she helps herself to a fiver out of his wallet. He can do things; he knows how to snatch fish out of a pond with his bare hands. And the bedroom action between Joanne and him is good—we can hear that through the thin walls and see it at the orgiastic party Joanne hosts. Fassbender, of Inglourious Basterds and Hunger, is making fast progress to A-list leading-man status.

Fassbender is extremely good-looking with his shirt off, and he has a soft accent that almost sounds like a New Yorker's (I doubt if he'll need any vocal coaching to do American films). All this is gossip compared to what he really has: a serious actor's humor and humanity. He's taking a risk, too, playing a character many would be ready to condemn as a molester.

But bravery is what marks Arnold, too. Only a female director would know how to chart the mixed emotions behind such a flirtation, or know how to make the sexual tension erotic instead of creepy, or to be able to make it clear what a young girl like Mia might think she wanted, while really not knowing what she might get. Weirdly, because of the film's finale, critics have been likening it to An Education—a much more distanced and refined piece of work. To use a metaphor from Fish Tank, the difference between these two movies is like the difference between a live gasping fish and a box of Mrs. Paul's.

Katie Jarvis was a nonprofessional picked off a railway platform by Arnold, and she is chillingly good. It also helps that Jarvis isn't a trained dancer. Although the arts can be a pathway out of the slums for kids, films rarely deal with those who have artsy dreams and loads of energy but only a bit of talent. It comforts the mass audience to believe that sheet talent will always have its way. We can see the neighborhood girls busting their moves; some of them are even better at it than Mia. One girl practices that walking-up-the-wall move that Donald O'Connor did in Singin' in the Rain. Arnold stops to watch some teens in a hallway doing their music-video dances: harem girls in search of a pasha, one with a navel jewel lolling out of her plump belly like a tongue.

There's nothing delicate in this world: the concrete, the highways encircling the project, the TV spilling nonstop bilge. Mia's mother is as negligent as an animal, and yet she's a real presence, too: she's true to her own nature. What amazes me most about Fish Tank, finally, is its raw hedonism. It's rare that a director who has such a sure eye and ear and such sensitivity can understand the bliss of a hard party.

FISH TANK, directed and written by Andrea Arnold, photographed by Robbie Ryan and starring Katie Jarvis and Michael Fassbender, opens Friday at the Nickelodeon.

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