Filmmaker Martin “Marty” Di Bergi is creating a documentary that follows the English rock group Spinal Tap on their 1982 United States concert tour to promote their new album Smell the Glove. The band comprises childhood friends David St. Hubbins and Nigel Tufnel on vocals and guitar, bassist Derek Smalls, keyboardist Viv Savage, and drummer Mick Shrimpton. They found early success as the Thamesmen with their single “Gimme Some Money”, before changing their name and achieving a minor hit with the flower power anthem “Listen to the Flower People”, and finally transitioning to heavy metal. Several of their previous drummers died in strange circumstances: spontaneous human combustion, a “bizarre gardening accident”, and choking on someone else’s vomit. Segments of Marty’s film show David and Nigel to be competent but dimwitted and immature musicians. At one point, Nigel shows Marty a custom-made amplifier that has volume knobs that go up to eleven, believing this would make their output louder.
Several of the band’s tour shows are canceled because of low ticket sales, and major retailers refuse to sell Smell the Glove because of its sexist cover art. Tensions arise between the band and their manager Ian Faith. David’s girlfriend Jeanine, a manipulative yoga and astrology devotee, joins the group on tour and participates in band meetings, influencing their costumes and stage presentation. The band’s distributor opts to release Smell the Glove with an entirely black cover without consulting the band. Despite their manager convincing the band that it would have a similar appeal to the White Album, the album fails to draw crowds to autograph sessions with the band.
Nigel suggests staging a lavish show, and asks Ian to order a Stonehenge megalith. However, Nigel, rushing a sketch on a napkin, mislabels its dimensions; the resulting prop is only 18 inches high, making the group a laughingstock. The group blames Ian, and when David suggests Jeanine should co-manage the group, Ian quits. The tour continues, rescheduled into smaller and smaller venues. Nigel is marginalized by Jeanine and David. At a gig at a United States Air Force base, Nigel is upset by an equipment malfunction and quits mid-performance. At their next gig, in an amphitheater at an amusement park, the band finds their repertoire is severely limited without Nigel, and improvise an experimental “Jazz Odyssey”, which is poorly received.
At the last show of the tour, David and Derek consider exploring old side projects, such as a musical theatre production about Jack the Ripper. Before they go on stage, Nigel arrives to tell them that their song “Sex Farm” has become a major hit in Japan, and that Ian wants to arrange a tour there. As the band performs, David invites Nigel onstage, reuniting them, which excites everyone but Jeanine. With Ian reinstalled as manager, Spinal Tap performs a series of sold-out shows in Japan, despite the loss of drummer Mick, who explodes onstage.