A good movie about a very bad one, Michael Paul Stephenson’s documentary follows the cult popularity of “Troll 2,” a 1989 horror flick that sank like a stone when new but has become an unlikely phenomenon among a small group of fans who relish its incredible—and unintentional—incompetence. It’s an affectionate tribute to what’s often called le bad cinema and its enthusiastic aficionados.

Stephenson was himself the kid star of “Troll 2,” which was filmed in Utah by Italian director Claudio Fragasso, but though he appears in the picture, the focus is actually on others—Fragasso and his erstwhile adult star George Hardy, an Alabama dentist who’s the first to get involved with the unexpected adulation of the fans. Hardy, an ebullient fellow who’s a beloved fixture in his hometown, revels in the attention he gets at special screenings, happily accepting the fact that the movie has become a camp classic and himself an oddball celebrity.

But we get a different take from Fragasso, who travels from Europe when he’s told of his old movie’s new success and is shocked when, as he puts it, the “crazy” audiences laugh at both what was supposed to be funny and what wasn’t. He also repeatedly “corrects” the recollections of cast members at Q&As, suggesting not just that their memories are faulty but that they don’t recognize the quality of his picture, which was designed—as his screenwriter/enabler remarks at one point, as a serious statement about vegetarianism! Fragasso is a pathetic character who repeatedly insists on his professionalism and can’t accept the fact that “Troll 2” is shlock—the guy’s oblivious to its awfulness.

There’s also poignancy to Stephenson and Hardy’s attempt to lure their co-star Margo Prey, who’s become a semi-recluse as she cares for her elderly mother, to the screening circuit. Their visit with her, replaying a famous scene from the movie as her mother looks on uncomfortably, is a sad portrait of the failure of an ambition to be a real actor (something one finds in the sketches of other cast member, too—a retired man, and another with mental problems). Happily, there’s a balance in that others have achieved a measure of success, though not always in thespian circles.

There’s also recognition toward the close about the fleeting nature of camp celebrity, when Stephenson and Hardy take their “Troll” campaign to comic book and sci-fi conventions where they barely register a blip on the radar of hard-core fans addicted to much bigger cult fish than Fragasso’s puny minnow.

But “Best Worst Movie” doesn’t go all teary and dark. It emphasizes that most of the people associated with “Troll 2” have happily accepted it for what it is and enjoyed the weird sort of celebrity it’s brought them, however temporary it might be. And since the picture is made by one of them, it avoids the air of condescension that could have made it seem mean-spirited. It might have offered more insight into the fan culture that embraces movies like this—taking a tack from “Trekkies” and “King of Kong”—but perhaps that’s asking the picture to be something other than its makers intended.

As one might expect from its subject, “Best Worst Movie” isn’t the slickest cinematic product—it has a homespun, unaffected quality, and gets repetitious at points (some of the “Troll” dialogue repeated for the adoring crowds—especially Hardy’s line “You don’t piss on hospitality!”—is heard far too often). But its very raggedness adds to the charm. When it’s over, in fact, you might be moved to search out “Troll 2” and see just what all the fuss is about.

Don’t say you haven’t been warned.