Why is Anvil, the storied Canadian heavy-metal band that's been in business for three decades, playing an almost empty dungeonlike club in Prague, when a band of its stature should be packing venues that hold a thousand people or more?

That question is asked in Sacha Gervasi's superb rockumentary Anvil! The Story of Anvil, by a British lawyer who's an avid Anvil fan. And drummer Robb Reiner does his best to answer succinctly. "I can answer that in one word," he says. "Er, two words." Pause. "Three words. We haven't got good management."

So it goes in the tale of two lifelong headbanging buddies, Reiner and guitarist Steve "Lips" Kudlow, who have kept a promise they made when they were 14 years old to keep rocking forever, despite a lack of encouragement from the music business and the population in general.

Anvil! The Story of Anvil can't help but evoke This Is Spinal Tap, the classic 1984 mockumentary directed by the other Rob Reiner, in terms of both sheer hair-metal ludicrousness and indignities suffered by a not-so-successful rock band on the road. Only these amps really do go up to 11, and Kudlow and Reiner, who aren't above physically attacking each other in fits of petulant rage despite decades of friendship, actually go to visit Stonehenge rather than have a miniature replica of it lowered onto the stage.

In their 1980s heyday, when Kudlow performed wearing a bondage harness, and using a dildo as a slide on his Flying Vee guitar, Anvil toured with bands like Bon Jovi, Whitesnake, and Scorpions, all of which would go on to sell millions of records.

Luminaries such as Lars Ulrich of Metallica and Slash of Guns N' Roses testify to the band's excellence and importance, but even the praise tends to come off like a bad joke. Scott Ian of Anthrax remembers listening to Anvil and thinking, "If we can't be better than that, we might as well go home."

What turns Reiner and Kudlow into real-life heroes - or at least real-life Canadian metal versions of Mickey Rourke's never-say-die lug in The Wrestler - is that despite a stunning lack of success, they never gave up. "It's over," Kudlow's sister, Droid, says at one point, lamenting Anvil's decision to keep making music when nobody wants to hear it. "It's been over for a long time."

But for Anvil - and particularly for Kudlow, for whom "music lives forever" - it's never over. And the opportunity to seize the day continues to present itself in this deeply human doc in the form of another shrieking guitar solo or chance to belt out a howling chorus of "Metal on Metal" at a Japanese rock festival.

"Time doesn't stop," Kudlow tells the camera. "It moves forward. Your belly gets big, your hair falls out. Twenty, 30, 40 years go by. You've got to do it now!"