Around 40 years after the original film, Martin "Marty" DiBergi (Rob Reiner) creates a documentary of the reunion/final show of the legendary heavy metal band Spinal Tap, including original members David St. Hubbins (Michael McKean), Nigel Tufnel (Christopher Guest), and Derek Smalls (Harry Shearer). He visits Hope Faith (Kerry Godliman), the daughter of the band's original manager Ian Faith, and finds that she inherited a contract requiring the band to perform one more concert. Marty goes on to reunite with Nigel, the band's former lead guitarist, who now manages a cheese-and-guitar shop with girlfriend Moira (Nina Conti) and plays guitar in a local folk band.Marty then goes to David, the band's former guitarist/lead singer, who produces music for true-crime podcasts and on-hold phone music. David's ex-wife, Jeanine Pettibone (June Chadwick) has become a nun. Marty finally goes to Derek, the band's former bassist, now the curator of a glue museum (Derek gets a glue-bottle stuck up his nose), who has composed a symphonic work called "Hell Toupee". Although tension exists between David and Nigel, the trio agreed to perform once more, though they need a drummer, as all their previous ones mysteriously died. They discuss how although the band has not performed for 15 years, interest into their music has increased after a video of Garth Brooks and Trisha Yearwood performing their song "Big Bottom" went viral.Finding a sleazy promoter who cannot comprehend their music, Simon Howler (Chris Addison), the band voyages to New Orleans, Louisiana to practice; their concert will fully occupy an arena slot vacated by Stormy Daniels. Questlove, Chad Smith and Lars Ulrich turn down offers to be drummer; auditions go badly, but eventually, a spirited young drummer, Didi Crockett (Valerie Franco), successfully auditions. Keyboardist Jeffrey "Caucasian Jack" Vanston also comes on board. However, original manager Bobbi Flekman (Fran Drescher) reveals that the band stressed her so much that she became a Buddhist; public relations man Artie Fufkin (Paul Shaffer) has become a used-car salesman.Living in a "ghost house" tourist attraction, the band must endure being served a whole-alligator supper while tourists with "ghost meters" wander through. It is revealed that the Rock and Roll Hall of Fame rejected the band's membership with a letter saying "Fuck off, sincerely yours"; Hope and Simon propose that the band establish their own hall of fame, the design for which resembles an IHOP site, featuring themselves. Nigel shows Mary a small cavity into his guitar, containing a piece of cheese and a grater. As the band reflects on aging and death, Derek composes a song called "Rockin' in the Urn". Rehearsals are distressing, as David and Nigel fail to connect musically. However, Sir Paul McCartney drops by and offers some helpful advice, later informing Marty that "Big Bottom" is "almost literature". Sir Elton John also drops by, though Simon derives him, and sings the band's song "Flower People", and agrees to sing "Stonehenge".Complications abound; the stage manager makes a huge model of a woman's bottom for "Big Bottom", which loudly emits flatulent sounds, annoying the band. Derek makes an unsuccessful pass at Didi, who promptly calls her beautiful girlfriend into the room. Simon even attempts to make the band exercise with an annoying personal trainer. as he wants them to dance like a K-pop boy band. Earlier, he had proposed that at least one of the band perish in concert, to produce a financially-valuable tribute opportunity. Simon then abandons the band, claiming he must visit his birth mother. Finally, David accuses Nigel of adultery with Jeanine, which Nigel denies.Frustrated, David takes a walk in New Orleans, seeing a Black bluesman singing a song which reminds him of the good old days with Nigel; he then informs Nigel he forgives him, though Nigel again pleads his innocence. As the concert finally begins, the crowd cheers the band's performance of their original singles. However, as "Stonehenge" unfolds, with Elton John at the piano, a huge stone monument descends from the ceiling, teetering precariously on the edge of Didi's relocated drum platform. Two little people in druid robes knock down the prop by accident, which crushes the piano, Elton, and the trio. Elton angrily screams "Fuck Spinal Tap!" The four recover in a hospital room. Derek later informs Marty that he was the one who had an affair with Jeanine. The final scene shows Marty at a restaurant, congratulating Didi on being the band's only surviving drummer; as she smiles and eats her healthy fruit-and-nut diet, she chokes, and Marty gives her the Heimlich maneuver, with a freeze-frame leaving the outcome uncertain.
Storyline
The band reunite after a 15-year break for one final concert.