A film about the Pixies? In another time, in another place, that might have been a commission made for Bunuel - an eyeball-slicing story of love, hate and obsession, with the part of the bassist played in alternating scenes by both Kim and Kelly Deal. Or maybe David Lynch: the tale of the stormy neon night when Black Francis transformed into Frank Black and traded in burgeoning grungey fame for solo obscurity.

But right here right now, the Pixies story has been told, perhaps even more perversely, as the strangely affecting tale of four bandmates washed up onto the shore of middle age, forgoing recrimination and regret for the opportunity to regroup, rehash and finally cash in on being ahead of their time.

Cantor and Galkin's film of the band's immensely successful 2004 reunion tour has been trailed elsewhere as some sensationally lurid backstage soap, complete with epic conflict, grand neuroses and spectacular falls from the wagon. In fact, it's a subtle, sensitive record of a group of people struggling to figure out what they once achieved, what it meant, and how they might somehow start again. It's the kind of rock doc HBO-auteur Alan Ball might have drafted - the story of a surrogate family where no one communicates, everything is swept under the carpet, and the very real midlife horrors of life and death find odd correlatives in the dream-logic of song.

Kelly Deal, along for the ride to keep an eye on her errant sis, shakes her head at one point and says "I've never seen four people not be able to talk to each other like you guys - you are the worse communicators ever... EVER!". So there's a lot of footage of backstage boredom, awkward small talk and nervy silence. Seldom have rock stars seemed so awesomely gawky. At the start of the tour Momma Deal remarks that touring will be good for the freshly rehabbed Kim - it'll get her out of the parental home where all she does is "sew and make paper snowflakes". Drummer David Lowering, meanwhile, is sleeping on couches, struggling to make a go of his science/magic stage show and supplementing his income by beachcombing with his metal detector.

Though the film-makers' reticence is sometimes frustrating - the only time Thompson is really questioned is via footage of an NME phone interview - it does allow the story of the reunion to develop naturally, from the group's initial wariness through their touching awe at the audiences they're attracting ("I was fucking FREAKING OUT out there!" beams Kim after the first show), the realities of trying to raising a family while on the road, up to the moment when, increasingly distressed by his father's mid-tour death, Lowering loses it onstage in Chicago.

But the real heart of the film lies elsewhere: in Lowering's pride that he was able to salute his father from the stage in London, in the scenes of Deal being mobbed by squealing teenage girl devotees in Iceland and Chicago, in Thompson's continuing insistence that he's "still in the songwriting biz, y'know, if the band ever wanted to book some studio time", and ultimately in the shots of the band on stage after their last song in New York, peering out and waving to crowd in wonder, relishing the moment and even, as they finally leave the stage, hugging each other. The screeching, screaming, dark paragons of 1980s avant-primitive rock and roll wind up, 20 years, on being heart-warming. Now that's surreal.