Let's say it's 1984, and you've made a perfect movie. It's a new kind of documentary — a "mockumentary," you cleverly call it — and it takes aim at the twin idiocies of the rock-music business and the oncoming "reality" video age that had been heralded a decade earlier by (among other things) the PBS series "An American Family." Let's say your movie covers all the relevant bases in these two worlds, from overblown concert footage and the annoyance of amplifier volume limits to moronic interviews with self-infatuated rock stars and the insufficiency of dressing room hospitality platters (there's an infuriating cold-cut sandwich problem). The picture is greeted with puzzlement at first, but over ensuing years it slowly grows into a classic on the new home-video market.

However, having said pretty much all there was to say about its subjects, your movie looks like a standalone achievement. What point would there be to a sequel? What laughs would be left to mine?

The answer to that question, just now arriving, 41 years later, is "Spinal Tap II: The End Continues," a picture that rejoins its hapless characters — guitarists David St. Hubbins (Michael McKean) and Nigel Tufnel (Christopher Guest), bassist Derek Smalls (Harry Shearer), and director Marty DiBergi (Rob Reiner) — as they shamble out of retirement for one last, contractually mandated gig. The picture seems intended as a sweet (and probably final) farewell to this beloved crew, one that at the very least won't scuff the luster of the first "Spinal Tap." Sadly, in this it does not entirely succeed.

One of the movie's problems is its plot, which is even looser than that of "This Is Spinal Tap." The band is preparing for its one-off reunion concert at New Orleans' Lakefront Arena, and we spend quite a bit of time watching them rehearse. This involves the group playing some new tunes, which are fine, and also some Tap classics enhanced by conveniently available guest stars. Paul McCartney sits in on "Cups and Cakes," and Elton John joins a reprise of the Tap's 1967 hit "(Listen to the) Flower People," as well as a chaotic rendition of "Stonehenge" (with the titular model megalith, once ridiculously tiny, now all grown up). Elsewhere in cameo land, Garth Brooks and Trisha Yearwood, of all people, take on the mighty "Big Bottom," and do it justice, too.

What's missing from much of the picture are the sort of dizzyingly funny (and generally improvised) gags that were so abundant in the first movie. Some of the attempted zingers here just don't land. Derek's retirement project, for example, is a "glue museum," which doesn't make a lot of sense even when you see it (and it's accompanied by a physical bit that doesn't work either). There's also a dated Stormy Daniels reference that withers upon utterance, and a whole character — an exercise coach, played by the stalwart John Michael Higgins, who's been hired to get the band members in shape — who could be excised from the movie with no loss in narrative coherence.

Both the best and the worst moment in the movie concern the latest in the Tap's long line of drummers, most of them tragically deceased. This new one, a young woman named Didi, is very much alive, and she's played by an actual drummer — an actually great drummer, in fact — named Valerie Franco. Unfortunately, just as Franco is bringing some voltage to this boomer-centric tale, Derek Smalls, several decades her senior, moves in to hit on her. Alas for him, it turns out she already has a girlfriend.

The movie's unavoidable age factor — most of the actors are in their seventies; Shearer is 81 — imparts a melancholy mood to the proceedings. Could there really be an arena's worth of twentysomething fans for these geezers like the ones we see in the concert scenes here? Could this band really have a future? Or is it too late, at last, for another sequel?

Photos courtesy of Castle Rock Entertainment