When Jack Black sings in the mockingly infectious goof-rock duo Tenacious D, he drinks deep from the well of classic rock & roll style, and you can hear all the vocal gods he's channeling Axl Rose and his banshee wail, Ian Anderson and his foppish enunciated trills, Freddie Mercury and his vibratory soar, and, more than those, the so-passionate-it's-a-joke operatic jukebox bombast of Meat Loaf. (It's from vintage '70s Loaf that Black cops his popped eyes and matching quivery head shakes.) What makes this one-man mixtape of rock history come to life is that Black is more than just an imitator. He's still hooked on the teenage muse of his bedroom mirror, as he chases down every riff that pops out of his blissed-out id-brain. He's funny not in spite of his sincerity but because of it.
At its loopy, dumbass, we-don't-need-no-stinkin'-budget best, Tenacious D in The Pick of Destiny works just like a Tenacious D song. The movie feels giddy and eruptive, dopily enthralled with itself, and more or less made up on the spot, especially when it lapses into pure rock opera: Meat Loaf, as the hero's scolding dad, in an opening number that might have come out of Tommy; or Black, known as JB, watching his future partner Kyle Gass strum classical-gas guitar on the Venice Beach boardwalk, then adding improvised lyrics to Beethoven's ''Für Elise'' (''If you think it's time to f---in' rock...!'').
Then again, we also have to endure Tenacious D's quest for ''the pick of destiny,'' a Lord of the Rings guitar pick that legend has it is the tooth of Satan. It's a holy-grail pursuit that keeps stopping the film in its tracks. The Pick of Destiny has its tasty-lick moments, like the duo's sensationally obscene performance on open-mike night, Ben Stiller's cameo as a paranoid hippie guitar-store clerk, and a musical showdown with the giant red devil himself. But Black, a true star, and Gass, who squeezes more faux-star wattage than you'd expect out of his complete lack of star quality, are stuck in a movie that could have used a little more ragtag trippiness and a lot less excellent adventure.