What's the expiration date on a promise made when you're 14? That's when Steve Kudlow and Robb Reiner swore an oath to rock together forever, and sealed the pact by writing a song about the subject they were studying in history class. "Thumb Hang," their ode to the Spanish Inquisition, was the beginning of a four-decade partnership that became Anvil, Canada's answer to Slayer, Megadeth, Anthrax and Metallica.

By 1984 they were filling Japanese stadiums and palling around with hair-band legends. Then tastes changed, time passed, and Anvil's fan army dwindled to a few nostalgic holdouts. Now on the wrong side of 50, they're still thrashing in half-empty bars with a diehard dedication verging on heroism.

"Anvil! The Story of Anvil" is a real-life "Spinal Tap," minus the mockery. Directed by their onetime roadie Sacha Gervasi (who went on to write "The Terminal" for Steven Spielberg and Tom Hanks), it's an affectionate portrait of performers starving for applause, achingly funny as only real life can be. Gervasi has a fan's appreciation for his subjects. Rock notables such as Lars Ulrich of Metallica and Slash of Guns N' Roses appear and confirm the band's originality and importance. Kudlow and Reiner aren't numbskulls in Gervasi's eyes, and the film will bring most viewers around to his point of view. In the face of seemingly insurmountable odds, Anvil's iron men are willing to try harder than anyone else. They're achingly sincere, without a trace of hipster snark or decadence. It's a classic underdog tale: "Rocky" with Flying Vee guitars.

What keeps Anvil's quest from curdling into a lifelong vanity project is their joy in performing. Singer-guitarist Kudlow comes alive onstage, head-banging with a goofy grin, sheets of hair snapping around his bald patch. Reiner, a jeweler's son (no relation to "Spinal Tap" director Rob Reiner), proudly displays the drumstick necklace his unswervingly supportive father made him. Their families, however, are politely exasperated. "It's over," Kudlow's sister, Droid, declares. "It's been over for a long time."

Gervasi edits with a screenwriter's discerning eye. Kudlow, filling in between gigs as a food service worker in a hairnet, could be the little brother of "The Wrestler's" deli-clerking grappler. An ill-planned European tour becomes a comedy of errors, and their Swedish promoter is an aging rock chick with an incomprehensible accent. Trying to make herself understood over the phone, she spells out her location haltingly: "A, for ass ... S, for Sodom ..." Explaining why Anvil hasn't caught the brass ring after 13 albums, Reiner distills the band's problem to "Three words: We haven't got good management."

You can't help being carried along on their journey. Your heart falls when they're booked into an arena that holds 10,000 and 174 fans arrive. They borrow a small fortune to mix a new album with a famed producer, whose dials actually do go up to 11. Their subsequent meetings with record executives are miniature tragedies. There are successes, too, as when their Euro-tour manager marries the lead guitarist.

Gervasi cranks the suspense like a master in the film's final minutes. A nibble of interest from Japan brings the band to a heavy metal event at a stadium that seats 20,000 ... at 11:30 a.m., hardly prime time for ear-piercing cacophony. The camera trails them as they walk toward the arena, tension mounting. I won't disclose what they encounter when they hit the stage, except to say it's a jolt of emotion strong enough to blow a 50-amp fuse. Even if you consider heavy metal music a weapon of psychological torture, you'll be enthralled by this life-affirming hymn to cockeyed enthusiasm.

Colin Covert • 612-673-7186