If you have no other reason to see “Spinal Tap II: The End Continues,” just know that you’ll finally have the opportunity to see their Druid set piece at its proper size — a tiny Stonehenge is funny, but a giant Stonehenge is rife with comedic possibilities.

Four decades after audiences got a look at one of the most disastrous musical tours of all time, documentarian Marty DiBergi (Rob Reiner) returns to the subject that helped make his name: the rock band Spinal Tap.

It’s been about 15 years since the group last performed together — and even longer since they’ve been culturally relevant — but Marty manages to convince David St. Hubbins (Michael McKean), Nigel Tuffnel (Christopher Guest) and Derek Smalls (Harry Shearer) to reunite for a final concert to send the band off into the sunset on a high note.

With new management behind them and a reasonable goal of only one performance together, the ensemble is hopeful that they can recapture their one-time glory — provided they don’t break up all over again thanks to their petty squabbles.

It’s hard to say who’s the true lead here. Between lead singer David, lead guitarist Nigel and the lifetime friendship, full of thorny-patches, that the two have shared is well-embodied in the dynamic between McKean and Guest.

Despite having worked together many times since we last saw these characters in “This Is Spinal Tap,” viewers should have no trouble believing these are two old men who truly hate each other with a battle of the narcissists on display that almost eclipses where they were 40 years ago.

McKean is delightfully exasperating as the typical frontman who assumes leadership at all times — And why would he not with his most exciting recent project writing telephone hold music? — while Guest is surprisingly more relatable this time around as the deliberately difficult guitarist, eager to make a return to the stage after years of monotony running a cheese shop.

Shearer splits the difference as their faithful bassist, who may not have as much to contribute to the sound of the band but always manages to keep the energy weird. In this case, he is the proprietor of a museum devoted entirely to glue.

Reiner doesn’t need to do much beyond letting this trio of mismatched misfits fight it out in front of him, but he has help in the form of Kerry Godliman as the overworked daughter of their original manager — who left her the band in his will — Chris Addison as their eager concert promoter who proudly states that he cannot comprehend music, and a variety of familiar faces who are happy to hop in front of the camera for a moment if not actually help make much happen.

As if you could forget names like Bobbi Flekman or Artie Fufkin…

Valerie Franco makes a welcome appearance as the latest victim to sit in the Spinal Tap drum kit — after almost every other percussionist in the industry flat-out refused that cursed position — but it will take more than her upbeat attitude to make this group function again…

The rock mockumentary that set the tone for an entire genre is not exactly one that was screaming for a sequel. After all, the appeal of the 1984 film is its sense of controlled chaos, free-flowing improvised dialogue and gleeful lampooning of the era’s music.

Big hair, bigger egos — and of course “Big Bottom” — live on in this presentation of “Where Are They Now?” for a band that both was and wasn’t a big deal in the real world.

The movie that inspired Guest’s oeuvre of deluded people talking at length about inane quasi-celebrity — “Best in Show,” “Waiting for Guffman,” “A Mighty Wind” — gets back to its roots of unbridled lunacy, albeit to a much more polished degree than its predecessor, likely due to Reiner being back in the director’s chair with all four leads given screenplay credit.

We’re not watching a musical group that’s being torn apart due to spending countless hours on the road and enduring endless mishaps. This time, the stakes are low but the resentment high as old grudges rear their ugly head among bandmates who weren’t great together in the first place.

Old footage mixed with new makes for a curious contrast as we see these guys at their prime — a generous description — and a decline that they fully acknowledge and even lean into.

The irony being that McKean, Guest and Shearer are a harmonious trio — literally and figuratively — and have remained just as talented after this long even if their characters are painfully rusty though clearly capable of returning to the music they created so long ago.

The sight gags are more meager than they were back in the 80s, with those effectively traded in for a parade of celebrity cameos, but where the chuckles are lighter, the emotional depth of their narrative rings truer.

Yes, they are the butt of the joke, but they are also far enough removed at this point from the rock god personas they created that it’s more satisfying to see them hash out their old hurts.

Because if Sir Paul can’t fix a relationship between old school chums, who can?

It simply can’t live up to what came before, but “Spinal Tap II: The End Continues” is a wholly agreeable bookend project that provides enough humor and heart to keep fans happy.

The amps may not be cranked up to 11, but it’s not like that imaginary level of loudness was real in the first place, right?