Part of the enormous fun in watching Jack Black and Kyle Gass play the demented rock duo Tenacious D -- the two-headed love child of Black Sabbath, Molly Hatchet and Iron Maiden -- is observing their pleasure in the roles. They're intoxicated with delight at being onstage, living out every self-deluded air-guitarist's dream of being a mike-melting heavy metal god. They're not in it for fame, perks or groupies. (Well, Kyle wouldn't mind a little stage-door love, but he's not lucky that way.)

What drives them is the sheer jubilation of pinning the audience's ears back with music louder, faster and tastier than the world has ever known, even if their fan base is 12 people in a bar. They're superstars in their own minds and nothing else matters.

Black and Gass (who looks more like a portly insurance adjuster than a raging metalhead) have been performing as the D for 15 years live and on TV, to loud cult acclaim. Their smartly written movie debut is as slick as you'd expect with all that preparation, and director Liam Lynch wisely opens up the story to newcomers unversed in the glorious lore of the D.

"Tenacious D in the Pick of Destiny" is a sort of rock opera concept album on film, giving us the band's origins in character songs and loopy comic vignettes, beginning with young JB's rocking rebellion against his overbearing dad. The casting is spot on, with Meat Loaf as the belt-snapping born-again buzzkill, and Troy Gentile, an astounding Jack Black Mini-Me, as his wayward son.

JB is not great with directions, so it takes a while for him to arrive and when he does he's the grownup Black. Street-corner troubadour Gass (quite a sight in flowing locks and a Renaissance Fair puffy shirt) teaches the eager newcomer the ways of rock. After all, despite his shabby apartment and spare-change cup, KG says he's a rock star, and that's good enough for JB, who regards his mentor with puppy-eyed reverence.

Even when he learns that KG is a big phony who gets rent money from his parents, JB remains loyal. They can pay the rent themselves if they win at the local open mike night. But the only way to guarantee a win is to acquire the one supernatural talisman that connects all supreme rockers. They must find the fabled guitar pick fashioned from Lucifer's tooth -- the Pick of Destiny! Their quest leads them from burgling the Sacramento Rock 'n' Roll Museum to the mouth of Hell itself, where they challenge Beelzebub himself (the Foo Fighters' Dave Grohl) to a rock-off.

Well-mannered families whose little ones loved Black in "School of Rock" should steer clear of this one; the humor is wildly profane. For trash-loving grownups, there's a lot to enjoy. The film is peppered with goofball cameos, including Tim Robbins as a bitter derelict who lost a leg seeking the Pick, Amy Poehler as North America's snottiest waitress, and an uncredited John C. Reilly as Sasquatch.

The plot pings along energetically, and if it rarely makes much sense, it usually finds the shortest distance between two jokes. Like a dumb but infectious rock song, you can't really justify loving the movie for any serious reason. It just gets under your skin and makes you delirious, and that's all the explanation it needs.

Colin Covert • 612-673-7186 • ccovert@startribune.com