Though it’s loosely based on the experiences of a real guy who was a cover-(or tribute-)band member plucked out of obscurity to become a member of Judas Priest, this take on the story from John Stockwell, an erstwhile actor turned writer-director (“crazy/beautiful” was his most recent effort in that twin capacity), draws a lot more on the conventions of TV and film than on the real world. The actual fellow, it should be pointed out, reveled in his new fame and continued with the group; Chris Cole (Mark Wahlberg), the hero of Stockwell’s story (directed by Stephen Herek), follows a different career trajectory, which just happens to be the Rags-to-Riches-to- Redemption formula so beloved of pseudo-documentary television and hoary old cinema soap operas. “Rock Star,” as the result is generically titled (though the original version, as out-takes over the final credit crawls show, was called “Metal God”), is basically “Behind the Music” meets “A Star Is Born.” It’s no less shamelessly perfervid than the one and even more mawkish and schmaltzy than the other.

That this unhappy fact isn’t entirely fatal to the movie is a tribute to Wahlberg, who seems relieved to be free of the constraints imposed on him as the stone-faced hero of “Planet of the Apes” and lets loose as the naif whose obsession becomes a reality. Wahlberg actually smiles here, and proves himself an engaging and sympathetic protagonist. He also looks good in the tight-fitting leather outfits he has to wear, and struts about the stage with the swagger of a true performer–not surprising, considering that he actually was one. He’s aided by the cinematography of Ueli Steiger, which captures his poses in the best possible light, and the editing of Trudy Ship, particularly in the stadium sequences, which are made to seem pretty authentic. The production and costume design by Mayne Berke and Aggie Guerard also help by catching the mid-eighties tone of shabby glamor and seedy glitter nicely, and the music, both Trevor Rabin’s score and the songs composed by such tunesmiths as Sammy Haggar, hits the right notes.

There are also some eye-catching supporting turns. The best comes from Timothy Spall as Mats, the slightly sleazy but considerate manager of Stone Dragon, as the band Chris is called to front is named. Spall, who’s worked with Mike Leigh on several occasions, is thoroughly convincing as a fellow with a load of regrets who still succeeds in keeping a semblance of order within the chaos of the music scene. Dominic West is also strong as Kirk Cuddy, the Dragon leader, who’s instrumental in picking Cole for the gig. The other guys who make up the two bands–some musicians, some actors–are good as well, though Timothy Olyphant is a tad affected as Chris’ best friend in the tribute band Blood Pollution (at times he strikes one as a younger Crispin Glover), and Jason Flemyng overdoes things as Bobby Beers, the original lead singer of Dragon, who’s removed partially because he’s been outed. Jennifer Aniston, meanwhile, is simply pallid as Emily, the girlfriend-manager who becomes increasingly distressed by her beau’s behavior once he grabs the brass ring.

But the real weaknesses in the picture are the fault of Stockwell and Herek. As the former has constructed the plot, the first half–dealing with Chris’ eager-fan phase–is enjoyable nonsense, even if his family (impossibly supportive parents and mean cop brother) is portrayed far too broadly and the crisis within his band is heavy-handed. But once he’s called to L.A. for an audition, the writing deteriorates. There’s a ridiculously sultry PR agent played by Dagmara Dominczyk as though she were a refugee from the “Rocky Horror Picture Show,” and a bevy of trophy wives and girlfriends of the Dragonaires so dumb-blonde that they make one wish that Judy Holliday were still around to show them how it should be done. Even worse, at this point the narrative gets abbreviated and frantic, so that we miss even a hint of the reckless enjoyment Chris must feel before he’s plunged into disenchantment with his role as a mere front-man for the group and the rampant sex-and-drugs in which his new colleagues indulge. The picture rushes from rise to fall without bothering to show us enough of the peak period that must have come between the two. (Herek doesn’t help by using crude devices like speeded-up film to suggest the hysteria of the touring life; that was old-fashioned in the days of silent film.) But even had the transition been handled more skillfully, the plot turn would have been a mistake. Having Chris go into the doldrums because the members of Dragons don’t want him to write his own songs is a glum cliche (as well as a cheap reversal of his utter devotion to their tunes in the first half of the flick), and the manner in which Stockwell portrays his passing on the torch to an audience wannabe who’s just another version of his earlier self is embarrassingly hackneyed. The sketchy account of his post-Dragon days, as he re-links with his old buddies and, of course, with Emily, is sentimental rubbish, too.

“Rock Star” thus gives us an hour of mostly high-spirited joshing about Chris Cole’s life as a fan, followed by an increasingly downbeat and dreary sermon about how the fulfillment of his dream proves less than heavenly. Whether Wahlberg’s energy and the silly exuberance of the first segment are enough to make the whole package palatable is a matter of personal taste. But if you choose to take a chance on it, please be advised that, as befits a picture about a “Spinal Tap” sort of heavy-metal band, “Rock Star” is extremely loud. Especially if you’re not used to such stuff, some cotton swabs for your ears might be a better investment than that large tub of popcorn.