The same things that make Jack Black an irresistible comedian are what make the pseudo rock band Tenacious D such an irresistible draw.

As an actor in such comedies as “Nacho Libre,” “School of Rock,” and his breakthrough, “High Fidelity,” Mr. Black has sported the same manic intensity of John Belushi, yelling, jumping, and convulsing — throwing his entire being into his material. And while his outbursts may seem immature, and at times even infantile, Mr. Belushi proved long ago that when someone’s working this hard for a laugh, it’s hard not to be intrigued — and more than a little charmed.

As the lead singer for Tenacious D — a guitar duo which, through its stage theatrics, improvised comedy, and flair for the absurd has developed a rabid cult following — Mr. Black is just as winning. He isn’t a rocker, but a dreamer — the hero of every music lover who’s spent more time dreaming about the rush of being a rock star than putting pen to paper in the hope of writing the next great rock anthem.

He’s the guy who thinks if he throws himself into his act, it’ll work; that if he acts the part of a rock star, or focuses all his mental energy, that the success, the fame, the girls, and the tune will all come to him without a problem. That’s why he talks about making the masterpiece, tasty guitar licks, and rockin’ riffs. He’s not just a fan boy of the rock stars, but a genuine rock star wannabe. He has all the work ethic but none of the talent to get there.

The movie arrives long after the birth and rise of Tenacious D, which began in 1997 as a skit show on HBO following the failures and fortunes of Mr. Black and his bandmate, Kyle Gass. As one might expect, it’s more a gift for fans than for newcomers. The real question come opening night is whether anyone who doesn’t know the story of Messrs. Black and Gass and their outrageously silly fictional band will appreciate the film’s sense of humor.

Odds are against it.

But as is, those who know a little something about who these guys are and what kind of shtick they’re slinging will have a ball.

You might call this the prequel to the D, or the story behind the band. Enter JB (Mr. Black), a child from an oppressively religious household, who runs away one day to find his music and his fame. Reaching the beaches of Hollywood, he finds KG (Mr. Gass) on a beach, playing his guitar for tips. JB’s jaw drops in awe as he fools himself into believing that he’s found the best guitar player in the world. Stoner KG lets the new kid “learn” from him until both realize they’re broke and set out on a quest to steal a magic guitar pick from the Rock and Roll Hall of Fame and pay the rent.

Desperation, though, isn’t the band’s weakness so much as a necessity. Everyone’s a little desperate in the movie — KG for rock immortality, JB for fame — and even the audience, outside the film’s fictional fantasy land, is desperate to root for two bums looking to make good.

Toward that end, everything in the film seems overdone and exaggerated. The drug trips drift from absurd to delirious; the operatic rock ballads that break out between the duo and the devil keep pounding away long after most films would have faded them down. Even the tarot cards that divide the film into its many segments are not throwaway segues, but elaborate, detailed tributes to the excessive exuberance of Tenacious D’s songs.

The film is as big and flawed as the band itself. But just as any concertgoer wants his favorite band to pull out all the stops, so too will every D fanatic want another hour of this rambling, pumped-up and psyched-out acid trip of a film. It’s so reckless, so absentminded, it’s addicting.

And I’ll take those flaws over a dancing, computer-generated penguin any day.