It’s easy to laugh at the members of Spinal Tap, whose bumbling misadventures, pompous observations and hackneyed sexual metaphors are so deliberately exaggerated, because they’re fictional. They were invented to lampoon the all-but-identical architects of an ’80s hair-metal subculture that had already ventured far into the realm of self-parody by the time the Christopher Guest-led trio arrived in 1984.

Watching the members of Anvil, a very real quartet of eardrum-shattering pioneers from that same era, isn’t as amusing, precisely because the humiliations and indignities they endure are depressingly real. Also in 1984, as young, leather-clad rockers, they seemed poised to make the leap to stardom, playing massive festivals with the likes of Bon Jovi, Whitesnake and the Scorpions.

Instead, something not so funny happened on the way to the L.A. Forum. Slash, one of several rock gods on hand to pay tribute in Anvil! The Story of Anvil, puts it best: “They should have made it a lot bigger. They never got the respect they deserved because as big an influence as they had on everybody, everybody just ripped them off and left them for dead.”

So begins Sacha Gervasi’s powerfully affecting new documentary about the band, with metal’s reigning royalty (others include Metallica’s Lars Ulrich, Lemmy Kilmister from Motörhead) paying long-overdue respects before the core members of Anvil – Steve “Lips” Kudlow and Robb Reiner, both working minimum-wage jobs between painfully under-attended gigs – are introduced.

They are down but determined not to be out, so long as even a handful of fans still attend their shows. And they still have stadium-sized dreams that, while unrealistic, might yet yield some modicum of success. Show business may be littered with tales of fizzled dreams and wasted youth, but hope springs eternals for these rowdy Canadians, whose ambition hardly dwarfs their very real talent.

Gervasi follows Anvil through a nightmare-laden tour of Europe, organized by a well-meaning but overmatched promoter and nearly sabotaged by miserly club owners. The results are often more wince-inducing than anything the other Rob Reiner dreamed up for This Is Spinal Tap, as the band plays to empty houses while internal tensions begin to rise. But thoughts of quitting are only briefly entertained.

The difference between The Story of Anvil and other landmark metal documentaries like The Decline of Western Civilization Part II is that its subjects are neither delusional egomaniacs nor willful antagonists. They are following a dream deferred by some 20 years, and still the friendship between Kudlow and Reiner persists, despite the occasional row. Theirs is an endearingly genuine passion, routinely discouraged by an industry that devalues its aging stars in favor of the bottom line.

Is Anvil worthy of the stardom Kudlow and Reiner aspire to? I’d say so. While they won’t be headlining Wembley Stadium anytime soon – given their surprisingly rabid following in the Far East, Budokan remains an outside possibility – the band’s sudden surge in publicity has already landed them a slot at June’s Download Festival. Gervasi doesn’t stick around long enough to document this happy ending, but even without it, The Story of Anvil leaves room for cautious optimism.

For one thing, its headbanging heroes can play. (Reiner is a virtuoso drummer.) When Kudlow, in desperation, mails an unfinished demo to a onetime producer best known for his work with Judas Priest and Black Sabbath, one might reasonably expect a polite brush-off, but the Hail Mary connects. It’s a small victory, but in a tale marred by frustrating setbacks and false starts, it feels much bigger than that.

There are other telling moments, as when Kudlow runs into fellow tour-circuit vets from Whitesnake and the Scorpions. They hardly seem inclined to reminisce about the glory days, but Kudlow, as much a wide-eyed fan as a fallen colleague, can barely conceal his excitement. He, like his bandmates, is without a hint of cynicism in his love of music, and it is that undying (albeit, in some ways, unrequited) love that makes The Story of Anvil so moving.

Are they the ultimate underdogs? Sure, and they still are. But one gets the feeling they’d rather die than lie down and accept it.