For those who’ve followed the band since their obscure HBO beginnings, the fact that there even is a Tenacious D in the Pick of Destiny comes as something of a miracle. Better still for fans is that they’ll be moderately happy with the results, as Liam Lynch’s origin story/road trip/heist musical has been crafted with the hardcore admirer in mind, as this willfully stupid adventure is populated with characters, gags, and one-liners borrowed from the D’s television and album catalog. Recounting the formation of the self-described “Greatest Band on Earth,” the film offers an introduction to the saga of JB (Jack Black) and KG (Kyle Gass), the portly, egomaniacal duo—emphasis on the maniacal in Black’s case—whose only goal in life is to shake the world to its very core with their Pentagram-and-Pot-powered rock. Beset by delusions of grandeur brought on by an excess of Black Sabbath listening and bong hits, the group, once formed, sets out to steal the legendary Pick of Destiny, which is made from the tooth of Satan and, as was previously the case with Jimmy Page, Eddie Van Halen, and Angus Young, grants the user untold guitar virtuoso powers. It’s a suitable setup for Black and Gass’s ridiculously dumb, dragon-and-demon and black magic-loving routine, and one that works best when Black is allowed to let loose with both profane tirades directed at his wide-eyed, long-suffering compatriot and, as in School of Rock, spastic, motormouthed ramblings about heavy metal’s majesty. Yet the inventive irreverence of the D’s prior work has here been replaced with more free-flowing, tossed-off skits that—as with a leaden sequence at a sorority house shindig—often go nowhere, and are only partially offset by loony moments such as Black’s psychedelic hallucination involving a flying Sasquatch. Cameos from Ben Stiller and Tim Robbins aren’t nearly as amusing and effective as the more subtle ones by Dave Grohl and John C. Reilly, just as the new D compositions—save for a blistering opening number that involves vocal accompaniment by Meat Loaf and Ronnie James Dio—don’t quite measure up to the band’s classical sauce tunes. Still, it’s hard to complain too much about this loosey-goosey romp’s patchiness when one of the rocking songs features the lyric “You’re gonna gargle mayonnaise”…and the reference isn’t to Hellman’s.