As a young man, Jables (JB) runs away from his strict, religious family and oppressive Midwestern town for Hollywood after being graced by the word of Dio on a quest to form the world's "most awesome" rock band. There, he meets acoustic guitarist Kage (KG), who is performing on the street, and begins worshiping him as a rock god. JB asks KG to teach him rock and roll, only to be refused. Later in the night, a homeless and crestfallen JB mopes on a park bench, seconds before being beaten up by mobsters. KG takes pity on JB and agrees to teach him, letting him sleep on the couch in his small apartment. KG feeds JB's fantasy by pretending to be famous with a self-named band ("The Kyle Gass Project"), exploiting him to do work such as cleaning his apartment and buying him weed, under the promise that JB can audition for his fictitious band. After JB learns KG is actually unemployed and living off his parents, the two become equal, and KG apologizes to JB by giving him a brand-new guitar. They create their own band: Tenacious D, named after matching birthmarks found on their buttocks.

Soon JB and KG learn the deepest secret of rock: all the rock legends used the same guitar pick, "the Pick of Destiny", which has 'supranatural' powers (a whole level above super). It was created by a dark wizard who'd summoned Satan for his own purposes, but was promptly attacked. A nearby blacksmith heard the commotion and distracted the demon by tossing a horseshoe at it, chipping its tooth in the process. As Satan was now "incomplete", the wizard was able to banish Satan back to Hell. To repay the blacksmith, who desired the heart of a fair maiden, the wizard fashioned the tooth into a pick that would give its holder unnatural prowess with stringed instruments. Infatuated by the prospect of becoming the next great rock star, JB immediately sets Tenacious D on a quest to steal the Pick of Destiny from the Rock & Roll History Museum. Along the way, the band briefly splits up; KG decides that sex comes first in "sex, drugs, and rock 'n' roll" after getting invited to a party by young women, while JB wants to stick to the mission at hand. They are eventually reunited after KG is kicked out of the party.

Later, after an intense chase from the museum security guards, the two manage to steal the Pick of Destiny from the rock & roll museum. Armed with this supernatural pick, they plan to use the winnings from a local bar's talent contest to pay their rent, but, before they can go on stage, they argue over who gets to use the pick first, snapping it in half accidentally. The bar's owner persuades them to go on stage and compete in the contest without the pick. The owner is revealed to be Satan in human form, who places the Pick of Destiny back on his broken tooth. Complete again, he obtains supernatural powers on earth and threatens to make Tenacious D his first victims.

To save their lives, Tenacious D challenge Satan to a rock-off, which under the terms of the "demon code" he cannot decline. The terms, suggested by JB, are that if Tenacious D win, Satan must return to hell and pay their rent, but if they lose, Satan can take KG back with him as a sex slave. After the duel, Satan deems his rock better and attempts to shoot KG with a bolt of lightning from his fingertips, but JB jumps in the way to save him, and the bolt bounces off the "JB KG" drawn logo on the base of his guitar, blowing off a piece of Satan's horn. As Satan is now incomplete once again, JB sends him back to Hell with the w…

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