Nothing beats the thrill of discovery at a major international film festival. Only a handful of agents in Berlin know about Metallica: Some Kind of Monster. A documentary about a heavy metal band, even one that has sold 90 million albums since 1991, is hardly steak and chips for the art-house predators who roam the market. Officially, Joe Berlinger and Bruce Sinofsky's film is not on the map. I'm the only critic in town who has been tipped off, and information like this is as reliable as a Chinese whisper.

"You better get in early," Jan Rofekamp, the sales rep for the film, says. Not for want of finding a seat, but because the opening blizzard of scenes in which the band are interrogated by brain-dead hacks is a masterclass in rock journalism. "You've been together for 22 years, eight world tours, and 11 albums. Give me one word to sum it all up," demands an interviewer. "One word to span our career?" asks the frontman James Hetfield. "What a f***ing stupid question."

There is a delicious sense that things will only get worse. Berlinger and Sinofsky were contracted to shoot the making of Metallica's new album, St Anger. What they capture is a band in fabulous crisis. There is nothing quite so exciting as watching a documentary turn into Spinal Tap before your eyes.

Metallica haven't released any new material or toured for years, and relations are at a low. On the eve of their first day recording at a deserted army barracks in San Francisco, the bass player, Jason Newsted, leaves the band. To stop their multimillion-dollar partnership unravelling completely, they hire a group therapist, Phil Towle, to help thrash out their personal issues at a mere $40,000 (£21,000) a month. The creepy " performance enhancement coach" sees lucrative years ahead. The band not only discover that they hardly know each other, but also quickly arrive at the notion that they might hate each other.

Petty jealousies become immovable grudges. Kirk Hammett's guitar solos are criticised as out of date. Lars Ulrich's drumming isn't "solid" enough. Towle's useless therapist has a field day. "He's under the impression that he's in the band," says an exasperated Hetfield. They are not thrilled by the mission statements Towle chalks up on boards, or the bits of paper stuck all over the studio with the word "zone" scrawled on them. But they are crucified by their fears, alcoholism and inadequacies. It slowly dawns on the band that this kind of documentary exposure is professionally insane. Conversely, no one wants to appear vulnerable by pulling the plug, so the film ploughs on like a runaway train. That, simply, is why it is such a sensation.

The needle match between Hetfield and Ulrich is the marvellously bitter heart. The two take swings at each other like spoilt brats. "You're just sitting here being a big dick," snarls Ulrich on Day 39. Hetfield storms out of the studio and disappears. For months.

Panic that Metallica might be well and truly finished seeps into the film. Six months later, the only member still going strong is Towle, whose therapy is having a spellbinding effect on his bank balance. Hetfield has been tracked down to a rehab clinic, but refuses to speak to anyone. We visit Kirk Hammett's ranch; and there is a reunion with Dave Mustaine, a former guitarist for Metallica (kicked out by a jealous Ulrich for being too friendly with Hetfield), who tells the guilty drummer that "people hate me because of you". It's priceless, comic stuff.

But is there life for this film without a band? With the sudden and dramatic return of a trimmer, abstemious Hetfield, a year after his departure, a story about the collapse of a world- famous group is dramatically transformed into a film about survival. It's an intoxicating piece of luck. Hetfield wants to finish the new record, but he is as unforgiving as ever. "You might look at this as a friend." he says, gesturing to the documentary crew. "To me, it's a beast." The shooting continues. As do the snipes.

Because of Hetfield's rehab commitments, the band can work only between noon and 4pm. When the frontman leaves the building, he doesn't want anyone listening to the music in case they make decisions behind his back. Ulrich explodes: "This is a f***ing rock'n'roll band. We're not meant to have rules. Don't tell me I can't listen to our music at 4.15 in the afternoon."

If the film has a lesson, it's that the exercise of control is the fault line in every democratic relationship. It's the Achilles' heel of rock music, and, arguably, civilisation itself. The band grapple with their differences as eloquently as a wet bar of soap. The rare joy is seeing multimillionaire icons do it in close-up. What's unexpected is how touching this public honesty can be. What's remarkable is how you end up feeling about these characters and, ultimately, their music. When Metallica finally test drive their new material live in San Quentin prison on Day 701, it brings a lump to the throat.

It's impossible to argue with the band's fairy-tale renaissance, or the documentary's concluding sweet notes. Towle is sacked. St Anger charts at number one in 30 countries. I still think that Ulrich and Hatfield will clobber each other until they go to their graves. But at least they've had the guts and decency to admit it.