‘Tenacious D’ keeps riffing as story thins out

If it is irresponsible to suggest that a mainstream movie might best be enjoyed in an enhanced state of consciousness, so be it.

“Tenacious D in the Pick of Destiny,” the big-screen incarnation of the Jack Black-Kyle Gass nightclub act-turned-HBO series, not only screams out to be a midnight movie, but one in need of, shall we say, an herbal supplement, and we aren’t talking ginkgo biloba.

It’s not that “Pick of Destiny” isn’t sporadically entertaining. Almost anything with Jack Black going full throttle, as he does here, is bound to have its moments of inspired lunacy. It’s just that when you have your wits about you, you start to care too much about things such as pacing and where your car is parked in the garage. None of which really matters if you’re truly down with the D, as the duo call themselves, and are willing to go the extra measure to enjoy the movie.

Black and Gass met as members of the theater troupe the Actors’ Gang and, taking their band name from sportscaster Marv Albert’s catchphrase for particularly intense defense in basketball, created an affectionate faux-metal parody with a surprisingly long shelf life. Playing shows since 1994, the duo have built a solid cult following and their success lies in the D’s passion for both the music they play and the music they parody, as well as the chemistry between the two men.

As spirited as the D’s live performances may be, the movie points out limitations as a long-form entity. Their brand of mischief played better in appearances on HBO’s “Mr. Show” and in their own shorts on the cable channel. “Pick of D” suffers from the same inability to go the distance as most of the movies derived from “Saturday Night Live” sketches. It runs out of gas (no pun intended) fairly early on, but not for lack of flatulence jokes.

As J.B., Black is the propulsive force of the group, providing evangelical fervor and impressively soaring rock vocals. Demonic eyebrows arched to capacity don’t hurt the impression that he’s the long-lost, corpulent offspring of AC/DC’s Angus Young. Those familiar only with the actor’s movie work, stretching from “Bob Roberts” to “High Fidelity,” “King King” and “Nacho Libre,” will recognize a kinship between J.B. and Black’s more grounded and fully realized character in “School of Rock.”

Counterpoint to J.B.’s playful embrace of grand rock gestures such as the power-knee slide is Gass’ classically quiet sideman, K.G. Balding and doughy, Gass looks like somebody’s dad who should be out mowing the lawn wearing Bermuda shorts, black socks and sandals -- except when he’s got his meaty hands wrapped around the neck of a guitar, expertly supplying the dead-on licks that give the music its requisite kick-butt swagger.

The movie starts off promisingly with a pre-title sequence featuring Lil’ J.B. (a strikingly cast Troy Gentile, who displays the same maniacally impish grin as Black) as he rebels against his father (an uncredited Meat Loaf) and turns to rock god Ronnie James Dio for advice. It’s a rockin’, audaciously operatic sequence that sets a breakneck pace the rest of the movie can’t hope to maintain.

The shaggy-dog plot lays out a fictionalized version of Tenacious D’s beginnings -- J.B. encounters K.G. playing Bach on the boardwalk at Venice Beach -- and then traces the pair’s pursuit of a magical guitar pick imbued with the power to make them the Greatest Band on Earth. The script, written by Black, Gass and director Liam Lynch, packs in gag after gag (and cameo after cameo), but there’s not enough dramatic impetus to keep it from becoming a drag.

kevin.crust@latimes.com

MPAA rating: R for pervasive language, sexual content and drug use. Running time: 1 hour, 37 minutes. In general release.