As a young man, Jack Black decides to break free from his strict religious family and the stifling environment of his hometown, Kickapoo, MO, embarking on an adventurous journey to Hollywood. His goal? To establish the most incredible rock band known to man. There, he encounters Kyle Gass, an acoustic guitarist strumming away on the streets, and becomes enamored by his talent, whimsically idolizing him as a rock deity.
In a twist of fate, Kyle, feeding Jack's enthusiasm, fabricates a story about his own fictitious band, "The Kyle Gass Project," and cleverly manipulates Jack into doing menial tasks for him, such as cleaning his apartment and procuring marijuana, with the false promise of an audition. When Jack eventually discovers that Kyle is actually living off his parents and is unemployed, their relationship shifts, leading Kyle to apologize by gifting Jack a shiny new guitar. This marks the inception of their very own band, Tenacious D, comically named after the birthmarks each man possesses; Jack's reads “Tenac,” while Kyle’s says “ious D”.
Their journey takes a compelling turn when they unearth a profound secret of rock music: all legendary rock artists once wielded the same extraordinary guitar pick, aptly named The Pick of Destiny. This mystical item, infused with "supranatural" powers, was said to be forged by a dark wizard from a fragment of Satan's tooth, bestowed upon a blacksmith who rescued the wizard from a formidable demon. The pick bestows its holder with unparalleled prowess in playing string instruments. Driven by the desire to ascend as rock royalty, Jack zealously leads Tenacious D on a mission to steal the Pick of Destiny from a museum dedicated to rock history.
However, challenges arise as the band momentarily parts ways; invited to perform at a party by some coeds, Kyle opts to indulge in the pleasures of sex, drugs, and rock 'n' roll, while Jack remains fixated on their primary goal. Their reunion comes after a disastrous solo performance by Kyle and an extravagant, life-threatening mushroom-induced hallucination that has Jack cavorting with the elusive Sasquatch through the woods.
Ultimately, they pilfer the Pick of Destiny, triumphing over the remarkably inept security at the museum, thanks to their marijuana haze. With this newfound power, they eagerly anticipate using their success from a local bar’s talent show to settle their rent. Yet, their greed leads to conflict, and in a moment of rage, they rupture the pick into two halves. After an uplifting talk from the bar’s owner, who unbeknownst to them is Satan in disguise, they step onto the stage, unaware of the impending doom that awaits.
Returning outside, having decided to share the pick, they encounter Satan, who regains his full powers upon placing the pick back on his damaged tooth. He menaces the duo, threatening to drag Kyle back to Hell. In a desperate bid to save their lives, Tenacious D daringly challenge Satan to a "rock-off," bound by the Demon Code. The stakes are high: victory means freedom and rent relief, while defeat would claim Kyle's soul.
Despite their best efforts, they fall short (as Kyle exclaims, “That was a masterpiece! / He rocks too hard because he's not a mortal man”), prompting Satan to unleash a devastating bolt of energy aimed at Kyle. In a heroic act, Jack leaps in front of Kyle, redirecting the energy to blow off a piece of Satan's horn. Seizing on Satan's imperfection, Jack conjures a powerful incantation to banish the dark figure.
In the end, they convert Satan's horn into the "Bong of Destiny," closing out their odyssey by lighting it up and writing new songs together.
