Tenacious D in The Pick of Destiny

If the rest of “Tenacious D in: The Pick of Destiny” were as funny as its first 10 minutes, it would be next to “Borat” as one of the year’s most riotous. In full rock-opera mode, a young Jack Black (Troy Gentile) battles with his father (Meat Loaf) over his guitar-playing, culminating in an appearance by Black Sabbath’s Ronnie James Dio. The dialogue is all sung, peppered with mystical allusions and self-serious progressive-rock goofiness. It’s a spot-on parody/homage, and Troy Gentile does a great Jack Black impersonation. (He played a young Jack in “Nacho Libre,” too. This is his destiny.)

The rest of the movie is uneven, ranging from the deliriously funny (like a crazy acid trip taken by one of the characters) to the belabored and overdone (like most of the elements related to a museum break-in). It’s a one-joke movie — sort of how Tenacious D is a one-joke band — but it’s a joke that can be pretty funny sometimes.

It certainly does not lack for energy. Written by Tenacious D (which is Jack Black and Kyle Gass) and Liam Lynch (of TV’s “Sifl & Olly”) and directed by Lynch, it’s an appealingly goofy flick. It seems at once pure-intentioned and filthy-mouthed, the two lead characters being grown-up children whose only desire in all the world is to rock. And I mean rawk!! Oh, and smoke pot. They want to do that, too.

Black and Gass play fictional versions of themselves — they go by JB and KG — who meet in Venice Beach, Calif., when both are aspiring guitarists. They form Tenacious D, a two-man band devoted to the rather obscure genre of acoustic heavy metal. Their songs are a mix of braggadocio, fantasy elements like unicorns and Sasquatch, and over-written metaphors. Like that opening sequence with young JB and his dad, Tenacious D walks the line between adoring the stylings of groups like Rush and Pink Floyd, and mocking them.

The guys have trouble making it big, though, and don’t have any money once Kyle’s mom stops sending him monthly checks. Then they learn of a magical guitar pick used by all the greats, a pick forged from the tooth of Satan himself. They set out to find it.

The film is in many ways a fully integrated musical, with characters bursting into song both onstage and off — which means Jack Black and Kyle Gass, without even trying, have made a rock musical that’s better than “Rent.” That’s gotta hurt.

Cameos by people like John C. Reilly and the Foo Fighters’ David Grohl keep things moving and offer some variety. Everyone seems committed to the silliness of the film, which is nice. A movie that calls itself “the greatest motion picture of all time” deserves at least a smile.

B- (1 hr., 33 min.; R, abundant harsh profanity, lots of vulgarity, sexual references, fleeting nudity.)