"8Mile" marks the screen debut of Eminem, the hugely successful white rapper who came up from the Detroit streets to prove that he has just as much credibility as his African American peers. He's made a fortune selling records of his catchy, sometimes brilliant rhymes, which have drawn millions of fans for their wit, raw emotion and poetic sophistication, as well as millions of detractors for their misogyny and homophobia. Whether you love him or hate him, you have to admit that Eminem (born Marshall Mathers III) has, in the parlance of his particular subculture, mad skillz.

But can he act? The answer is a qualified yes. Eminem is in every scene of "8 Mile" and holds his own with confidence and a prowling, growling physical grace. With his enormous eyes and the sort of aristocratic mouth that could have been painted in Renaissance Italy, he has the face of a bruised angel. It's impossible not to watch him, which is the prime factor in making a movie star. And for the final 15 minutes, viewers have the electrifying experience of watching Eminem do what he does best, improvising in a rap contest that constantly ups the ante in creative insults, personal invective and tribal provocation. But the payoff comes after an hour and a half of a long, criminally tiresome setup.

Eminem plays Jimmy Smith Jr., aka Bunny Rabbit, who has just broken up with his pregnant girlfriend and moved in with his mother (Kim Basinger) and little sister, who live in a Detroit trailer park. Rabbit works in a factory by day, trying to make enough money to make a demo record of his raps. At night, he and his friends Future (Mekhi Phifer), Sol George (Omar Benson Miller) and Cheddar Bob (Evan Jones) go to a local nightclub where Future emcees battles of the rappers. The movie opens just as Rabbit is about to enter his first battle: He jumps in place like a prizefighter. And once onstage, the contest is indeed like a boxing match: two gladiators fighting to the death, with only words for weapons.

This is a terrific, even inspiring, conceit, but director Curtis Hanson (who directed the overrated "L.A. Confidential" and the underrated "Wonder Boys") squanders the opportunity to make the "Raging Bull" of rap movies. Instead he opts for foursquare melodrama, taking viewers through the lumpenproletariat squalor of Rabbit's home life, a disastrous love affair with a would-be model (played by an authentically skanky-looking Brittany Murphy) and the rapper's tendency to freeze up when he's in front of an audience.

There's nothing surprising here and, what's worse, the stage-fright angle means that we see precious little of Eminem rapping in "8 Mile" -- a gambit that is frustrating to the point of torture. Hanson is trying to set up an almost-impossible state of tension before he reveals what Rabbit's been hiding all along, but it comes off as a big, disappointing tease. What's more, a movie that is supposed to celebrate verbal expression as an alternative to physical aggression has way too many fight scenes. For some reason Hanson has decided to make a rap movie in which fists fly more often than words do.

Eminem takes a sallow script by Scott Silver and makes the best of it: He has a terrific rapport with Phifer and his other friends, and he and Murphy create real sparks, although their sex scene in the middle of a factory feels protracted and gratuitous. Basinger's Georgia accent is accurate for the place (Detroit is filled with Southerners who migrated north for manufacturing jobs), but it still doesn't ring true; she looks and sounds exactly like a gorgeous Oscar-winning actress who's slumming.

That said, there are a few utterly on-point touches in "8 Mile" that the city's natives will recognize and appreciate. When Rabbit and his friends shoot a paint-ball gun out of their car, a police cruiser seems to give chase -- until they realize the cops have bigger fish to fry than a car full of boys shooting a toy gun. And the final rap battle is a thing of pure exhilaration, agility and beleaguered Detroit pride. In Rabbit's most hilarious putdown, he "outs" his competitor as a graduate of Cranbrook, a tony private school just outside the city.

This comes in the midst of a torrent of words during which viewers finally see why Eminem's such a hit: He has an expressive range -- physical, verbal and vocal -- that far outstrips the declamatory monotone of most of his peers. There's no doubt that Eminem has the talent and presence of a star. It's just a shame that the filmmakers didn't capture his power with mad skillz of their own.

8 Mile (111 minutes, at area theaters) is rated R for strong language, sexuality, some violence and drug use.

In his acting debut, the rapper boasts the talent and presence of a movie star. But his performance is nearly spoiled by the film's melodrama. Brittany Murphy as a would-be model and Eminem as a would-be rap star show real chemistry, though their sex scene comes off as gratuitous. Jimmy (Eminem) is rolling with his crew -- from left, Mekhi Phifer, Evan Jones and De'Angelo Wilson -- through the Detroit streets.