Talk about metal fatigue

When they met in the early 1980s, Metallica were acned California teens. Twenty years and 90 million albums on, they are the most successful heavy-metal band in the world. But when the metal gods hit middle age, they hit meltdown.

Metallica: Some Kind of Monster is the story of that meltdown and their group therapy with the "performance enhancement coach" Phil Towle. It was filmed over three years by Bruce Sinofsky and Joe Berlinger. And, unlike Spinal Tap, it's real.

"The reason the film feels so authentic is that it was completely unplanned," Berlinger says. "The band's management wanted an infomercial disguised as a documentary." Early in 2001, he and Sinofsky headed for the old barracks in Presidio, San Francisco, where the band planned to record their new album. While the film-makers prepared for a short commercial shoot, the band's singer, James Hetfield, shot bears in Russia, the guitarist, Kirk Hammett, rode horses on his ranch, and the drummer, Lars Ulrich, enjoyed his art collection. What the fortysomething "squillionaires" weren't doing was recording, playing or talking to one another.

Then the infamous Playboy interview at least got them talking. The bassist, Jason Newsted, mocked Hetfield's singing, Hetfield slagged off Ulrich's drumming, Hammett described how "Alcoholica" would "drink day in, day out". Smelling a break-up, the record company moved Towle in. On his first day, Newsted said Phil's mission statement was "fucking lame" and walked out, and the promo flick became something more interesting. "I never dreamt it would become this incredible document of the dark side of rock'n'roll," Berlinger says.

Newsted's going leaves the film firmly focused on the shouting, swearing, sulking power-struggle between the founders, Hetfield and Ulrich. After 20 years together, the burly redneck and baby-faced Dane bitch like a married couple. Hammett says little: Buddhism in action, it seems. "I try to be an example of egolessness," he says.

But Phil, with his yellow sweaters and psychobabble, is the star. "I'm afraid he's under the impression he's in the band," mutters Hetfield after two years of analysis. The hard men of Metallica have a secret meeting about how to bin Towle, but he won't abandon them - or his $40,000-a-month pay-cheque. Phil writes up inspirational sayings like: "Zone It!" When the band change the Z to a B, it's very Spinal Tap. The Tap comparisons "kind of bug" him, Berlinger says, because "these guys take this real emotional journey. The transformation of Hetfield on film is incredible. At the start he's a surly drunk, and by the end he's a very evolved person."

The film is packed with scenes so silly that they transcend satire. Hetfield needs a rhyme for "mine". Kirk and Lars chip in: "Step on a landmine?" "Rip out your spine?" Surely watching these rich rock stars go through therapy seemed ludicrously self-indulgent? "These guys were brave enough, after 22 years, to confront these issues - not for financial reasons but for creative and existential reasons. I found that inspiring," Berlinger says.

There was huge tension at the first showing. But, says Berlinger, "they laughed, they cringed, there was silence. At the end, they looked at us with this mix of bemusement and shock. And then they just left the room." Berlinger and Sinofsky feared the worst, but the band bought the footage and gave them the final cut.

And Phil's reaction? "Well, I'm human, and my ego gets affected. When I saw the film, I felt disappointed. My personality is oversimplified. Nobody likes for the completeness of their personality not to be represented."

When will Metallica be done with therapy? "I think that time is now," Towle says. "But in performance coaching, which is different to therapy, it's about excellence. I think there's always room for someone like me in that process."