What would Dio do?

Your reaction to Tenacious D: The Pick of Destiny probably will depend on whether you’re in on the joke to begin with. If you’ve been following the faux heavy-metal rock band formed by actors Jack Black and Kyle Gass in 1994—and if you’ve stayed loyal through their short-lived HBO series, their CD, and their wacky live club appearances—then you’ll already be on board with this strained-humor version of how the two met and formed (ahem) “the greatest band on earth.” If not, and you somehow find yourself in a theater showing the film, you may sit there wondering why someone would pay good money to watch what looks like a vanity home movie shot by Black, Gass and their pals in their spare time between real gigs.

The pals include director Liam Lynch (who receives co-writer credit with Gass and Black) and a number of actors who drop in for larky cameo roles—among them Tim Robbins, Amy Poehler, Meat Loaf and Ben Stiller (who is also credited as executive producer).

The movie opens with a pre-title sequence in which a younger version of Black’s character “JB” (Troy Gentile, who also played Black as a child in Nacho Libre) is beaten with a belt by his Bible-thumping, rock-hating father (Meat Loaf). After the credits, the story gets under way with JB meeting Gass’ character (“Kyle Gass,” a.k.a. as “KG” or “Kage”) on the boardwalk in Venice, Calif., where the latter is playing guitar for handouts. Gass, with a toss of his stringy shoulder-length hair, at first spurns JB’s admiring attention (“I’m workin’ alone here”), but in time the two establish a mentor-disciple relationship—even after Kyle’s hair turns out to be a wig and his supposed royalty checks for a song called “I Love You, Pumpkin” are really subsidies from his mother. Their partnership is cemented when they discover that they have matching birthmarks on their buttocks; JB’s says “Tenac,” while Kyle’s reads “ious D.” It’s like a mystical moment, man: Tenacious D.

In the duo’s pot-fueled ambition to become the biggest thing in hard rock since their idol Ronnie James Dio, they see something that has heretofore escaped notice—many of the great bands used the same kind of guitar pick. Looking for such a pick for themselves, they encounter a guitar-shop clerk (Stiller) who, shifty-eyed and suspicious, hisses at them, “How did you know about that?!” Taking the boys into a darkened back room and speaking low over the flame of a cigarette lighter, he tells them the story. All those bands didn’t use the same kind of pick; it’s the exact same pick: the Pick of Destiny. It was fashioned centuries ago from the actual tooth of Satan, he tells them, and is now kept in a display case at the “Rock and Roll Museum,” unnoticed and unheralded, just waiting for the right rocker to come along and claim it.

This sends JB and Kyle on a quest to invade the Rock and Roll Museum (a stand-in for Cleveland’s Rock and Roll Hall of Fame, apparently relocated to Sacramento so the boys can get there in one day) and steal the Pick of Destiny in time to come back to Los Angeles and win Open Mic Night at their neighborhood club.

The Pick of Destiny is the movie equivalent of the music Tenacious D makes. As they stand around on the boardwalk, or in their dingy apartment, or on the stage at the club, with Kyle riffing on his guitar and JB holding the microphone as if trying to wring its neck, they contort their bodies and twist their faces with the superhuman effort of making what comes out as perfectly ordinary guitar cords and vocal sounds. It’s a parody of rockers who make bad music with clenched faces and sweating brows, expecting us to respond to the exertion rather than the music.

The duo strains at their comedy the same way they strain at their music. Black performs in a manic, wide-eyed style that says, between the lines, “Man, you are gonna love this. This is just so—damn—funny.” And pudgy, bald, schlubby-looking Gass acting like a chick-magnet rock god must surely, dontcha think, be the most hilarious thing since … well, since The Blues Brothers, which is in many ways the model for Tenacious D—an affectionate parody of the kind of music the two stars love.