Michael Fassbender is in there somewhere. The Irish-German star of Shame and the X-Men movies spends most of this unlikely band comedy inside an oversized papier-mache head, which ought to make Frank the world’s worst musical frontman. Instead he inspires fierce, cultish devotion among his followers—which is to say his band, the Soronprfbs, for they may have no actual fans. Part of the suspense here for viewers is when or if Frank will ever remove his fake noggin. For new keyboard player Jon (Domhnall Gleeson), the suspense is whether Frank’s suspicious acolytes will ever truly accept him; and further, if Frank will ever acknowledge Jon as a musician likewise possessing genuine talent.

This is a fundamentally sad film, yet one full of slapstick, silliness, and laughter. Frank is essentially unknowable, so his band willingly accepts every humiliation and ridiculous challenge to earn—or at least guess at—his good favor. (The most hilariously protective of Frank, and scornful of Jon, is Maggie Gyllenhaal’s fierce Clara—a kind of muse and ninja.) Is Frank cult leader or charlatan, genius or insane? It’s hard to decide, since he never breaks character—or can’t, really, given the mask. (“I have a medical condition,” he insists at a border crossing; otherwise, like Jon, we’re left to wonder why he wears it.)

There’s a small grain of truth here: English journalist Jon Ronson (The Men Who Stare at Goats) really did play in a band led by a guy who wore a big fake Frank head. However, he and director Lenny Abrahamson have greatly embellished the tale, which now makes you think of any number of outsider artist-savants and the thrall they exert over their insecure followers. Frank is equally Jon’s story: from aimless post-collegiate songwriter to eager bandmate who begins promoting the Soronprfbs on YouTube and Twitter, which eventually leads to an invitation to the South by Southwest music festival in Texas. Jon’s ambition, checked at every turn by Clara, turns out to be not such a good fit with the group, which may not truly want to be heard. (Their songs are by Stephen Rennicks.)

Fine, you might ask, but what about poor Fassbender? How can he even act inside that blank orb? Well, by using his voice (he sings a bit like Ian Curtis), playing passable guitar, pratfalls, and passing out notes. (“Welcoming smile,” reads one.) It’s a stunt, but it’s an effective stunt: He’s withholding from us what Frank withholds from his band. The less he shows, maybe the more we like him. And conversely, the envious, ingenuous Jon becomes less attractive as the movie goes along. Perhaps the music snobs are right in the end: You ruin something good by sharing it. Opens Fri., Aug. 29 at Varsity. Rated R. 95 minutes.

bmiller@seattleweekly.com