Starring: Jack Black, Kyle Gass, Jason Reed, Ronnie James Dio, Troy Gentile, Dave Grohl. Rated R.

Tenacious D, the self-proclaimed Greatest Band on Earth, has risen from the humblest of beginnings – in this case, a bit part in the 1996 Pauly Shore comedy Bio-Dome that led to a short-lived HBO series. Loosed on a startled public, they recorded their eponymous debut, a gleefully raunchy collection of ribald ballads and odes to kielbasa, in 2001. Now, improbably, comes Tenacious D in the Pick of Destiny, which finds D frontmen Jack Black and Kyle Gass perfecting the art of cock-pushups, learning valuable life lessons from Ronnie James Dio and out-rocking the Dark Prince himself.

If that sounds preposterous… well, nobody’s arguing. Whether you appreciate The Pick of Destiny depends largely on your threshold for Black’s frenzied antics, the manic, wild-eyed silliness he brought to superior comedies like High Fidelity and School of Rock. Fans who relished the D’s HBO series, or unabashedly juvenile anthems like “Fuck Her Gently” and “Tribute,” will feel right at home in this raucous fantasyland, where ultra-lowbrow humor rules the day. Others may leave scratching their heads.

And rightfully so. The Pick of Destiny is a ragtag mess, and there are times when it sputters down the stretch, as if the D’s initial inspiration wasn’t enough to fill a feature-length film. Even so, the story, written by Black, Gass and director Liam Lynch, is pleasantly ludicrous. After meeting for the first time on the Venice Beach boardwalk, JB and KG embark on their quest for the most powerful artifact in rock history, the Pick of Destiny – a guitar pick fashioned from Satan’s tooth, known to inspire greatness in all who possess it. (Former beneficiaries include Eddie Van Halen and The Who’s Pete Townshend.) Together, the fledgling duo travels to the Rock and Roll Museum, determined to steal the pick for use at an open-mic night.

Along the way, there are plenty of sharp one-liners and big-name cameos, including John C. Reilly as Sasquatch and Dave Grohl as the Devil, who wants his tooth back. Perhaps the most inspired guest spots come early on, during the hilarious mini-rock opera sequence that opens the movie. There, a young JB (Troy Gentile, who played a similar role in Nacho Libre) rebels against his rock-hating father (Meat Loaf) and seeks spiritual counsel from Dio, who convinces him to embrace his inner demons and abandon the Missouri suburbs for the Hollywood hills.

One of the running gags in the D’s epic saga is their supreme self-assurance. Most bands would settle just to be mentioned in the same breath as AC/DC, Black Sabbath and Led Zeppelin, the kind of hard-rocking icons JB and KG clearly idolize. And while neither is anyone’s idea of a typical rock god – KG’s portly physique and absentee hairline don’t exactly scream MTV – the D know no self-doubt. They are peerless legends in their own mind, and that’s good enough for them. Not surprisingly, they’ve billed The Pick of Destiny as the Greatest Motion Picture of All Time. It’s not, of course, but it is a worthy enough distraction to make Dio proud.