IT'S BEEN 20 years since Robert Fishman (Rainn Wilson), the metal-head hero of " The Rocker," lost his chance to live the dream, and he's been half asleep ever since. Back in the days of striped spandex and mile-high hair, "Fish" was the drummer for Vesuvius, a band on the verge of making it big. But since his bandmates booted him on the same night they signed a record deal, he's gone from heavy-metal rocking to working a soul-crushing desk job, his head bowed in permanent defeat.

Living in his sister's unfurnished attic, where his lumbering frame frequently connects with the low roof beams, Fish has resigned himself to a life of failure. But when his high-school-age nephew (Josh Gad) needs a fill-in drummer for his band's prom gig, Fish is reluctantly coaxed out of retirement and shoved toward a second chance.

Although he hasn't updated his wardrobe since the 1980s, Fish isn't like the men-children who populate so many recent comedies. He's loud and lumpish and a little lost, as if there are secrets to life that everyone knows but him.

He's also, on occasion, simply oblivious. When Matt's band, which also includes a brooding singer (Teddy Geiger) and a sardonic bassist (Emma Stone), sets up a rehearsal via video chat, Fish takes the camera for a microphone and promptly strips off his clothes to drum in the buff. The footage, inevitably, ends up on YouTube, and before long, "the naked drummer" is the new Christian the Lion -- and fans start to pack their shows.

Director Peter Cattaneo ("The Full Monty") is well-versed in the redemptive power of male nudity, but he doesn't give "The Rocker" the necessary snap. Like its protagonist, the movie is sweet but slow and a little out of date. Given that their collective résumé includes " The Simpsons" and "The Larry Sanders Show," it's hard to believe writers Wallace Woladarsky and Maya Forbes couldn't come up with more pungent pop-cultural targets than "Titanic" and U2. Fish isn't the only one who seems like he's slept through the last decade.

But there's something pleasantly old-fashioned about the movie's leisurely pace (as well as its earnest believe-in-yourself message), which seems as much a relic of a bygone era as Fish's shaggy haircut. The romance between Fish and Kim ( Christina Applegate), the single mom who chaperons the band's tour, may be foreordained, but it doesn't feel forced.

On "The Office" and "Six Feet Under," Wilson has proved his genius for playing awkward, strangely lovable creeps, which is essentially all "The Rocker" asks of him. The proportions may be varied (more lovable, less creep), but there's no heavy lifting involved. Wilson is at his most animated when demonstrating what aficionados know as "drum face," his mouth stretched wide in a primal yell. But for the most part, "The Rocker" is content to simply keep the beat, marking time as the summer movie season moves on.

"The Rocker." MPAA rated: PG-13 for drug and sexual references, nudity and language. Running time: 1 hour, 42 minutes. In general release.