The only thing that inspired much optimism about the laugh-deprived trailers and television ads for "Tenacious D in The Pick of Destiny" was the knowledge that finally, the comedic rock-'n'-roll duo of Jack Black (2006's "Nacho Libre") and Kyle Gass (2003's "Elf") would be onscreen together, front and center. They have a grand chemistry with each other, perhaps because they are longtime best friends in real life, and even if the marketing campaign made their movie look like an unsavory cross between 2000's "Dude, Where's My Car?" and 2000's "Little Nicky," at least their very appearances would be worth something. And then the opening scene hit.

The first ten minutes of "Tenacious D in The Pick of Destiny" are brilliant, plain and simple, single-handedly squashing all initial skepticism from moment one and leaving a blaze of fire behind in the wake of what starts as a seriously high-powered, genuinely gut-busting R-rated rock opera. The early scenes introduce the viewer through song to Lil' JB (fabulous Jack Black doppleganger Troy Gentile), a passionate pint-sized rock-'n'-roller viewed as an anarchist by his stern father (Meat Loaf) and forced to run away from home and head to Hollywood for musical stardom. Once in L.A.—a trip that apparently takes twenty-five years as he travels to every town in the U.S. named Hollywood before reaching California—JB (now played by Jack Black) spots KG (Kyle Gass) performing licks on his guitar for money and is suddenly convinced that they are meant to rock together. So far, so good, and director Liam Lynch (2005's "Sarah Silverman: Jesus Is Magic") continues to surpass expectations with a howlingly clever confrontation between JB and the masked ultraviolent hooligans of "A Clockwork Orange."

And then, for the most part, the fun (and music) screech to a halt. If ever a motion picture this year got off to such a glorious start before spiraling out of control, it is "Tenacious D in The Pick of Destiny." The story proper isn't the problem, as JB and KG set off on the road for the Rock and Roll History Museum to snatch the legendary guitar pick of the title, born from one of Satan's teeth and accountable for the success of the top rock gods throughout history. Thinking that the pick will have the same effect on their struggling band, JB and KG run into squabbles, frat parties, a mushroom trip, a crazed, one-legged wannabe rocker, and literal road blocks along the way to the museum—and their very own destiny. These episodic skits, few of them taking advantage of their promise or comedic potential, wear out their welcome quickly. They simply aren't very funny or memorable, and the savvy, joyfully coarse dialogue of the first act is replaced by a haze of bong smoke and lazy plotting.

If anything, the film as a whole would work better for anyone under the influence of mind-altering substances. There is nothing of any consequence to grab onto or think about, but it's psychedelic and off-the-wall enough that it could still win over a legion of fans (read: potheads and recreational drug-users). For the rest of us with clearer vision, "Tenacious D in The Pick of Destiny" is pretty disposable, the biggest disappointment being that the musical inspirations of the opening are virtually dropped until the resuscitative climax where JB and KG compete against Satan himself for the sought-after pick. Had the entire picture kept on the path of the beginning section and been performed as a rock opera rather than a conventional, dumbed-down comedy with intermittent stylings from Tenacious D, the film wouldn't just be one of the best of the year, but also one of the most rousing screen musicals in years. Alas, it was not to be.

With nonstop blink-and-you'll-miss-them cameos (i.e. Amy Poehler, Colin Hanks, Amy Adams, John C. Reilly) and some extended appearances by the likes of Tim Robbins (2005's "War of the Worlds") and Ben Stiller (2004's "Meet the Fockers"), "Tenacious D in The Pick of Destiny" slowly but surely begins to run on desperation as it tries to retain audience interest with little of interest to be had. Adapting a band for a fictional big-screen adventure has and can be done well with the right innovative handling, but there is also the risk of being stretched too thin if the creative juices stop flowing. "Tenacious D in The Pick of Destiny" sadly falls into the latter camp, and neither writer-director Liam Lynch nor co-writers Jack Black and Kyle Gass exhibit the creativity to take the project over the edge. Black's and Gass' music is worth listening to—they are talented and passionate musicians, no doubt about it—but buying the soundtrack album and bypassing the inferior movie it is based upon is the potential viewer's best bet.