Some movies are built for mass appeal, and some aim smaller — "festival films," they’re called.

"Frank" feels like a "festival film" aimed squarely at one festival, the one that is the setting of its third act. Here’s an eccentric tragicomedy with music, built to play like gangbusters at Austin’s South by Southwest music-movie fanboy/fangirl festival.

The title character is a singer/songwriter who performs and lives his entire life wearing a gigantic plastic head over his skull. The fact that the great Michael Fassbender is the talking, fuming, rambling and singing man-behind-the-mask makes this wildly improbable film all the more intriguing.

Domhnall Gleeson is Jon, an office drone and aspiring songwriter whose banal observations of life-observed, set to music, aren’t getting him anywhere. And then, as he’s watching this keyboard player go mad and try to drown himself in the Irish Sea, opportunity knocks. Don (Scoot McNairy), the manager of the band (called Soronprfbs), asks Jon to fill in.

Their music is madness incarnate — wild trills of guitar, drums, synthesizer and theremin. Frank, his mask containing a built-in microphone, croons on about "screeching frequencies of pulsing infinity." The women in the group (Maggie Gyllenhaal, Carla Azar) are protective of Frank and enraged at the universe. Clara (Gyllenhaal) is violently toxic and takes an instant loathing to Jon.

"You are fingers being told which keys to push," she hisses, dismissively.

But Frank appreciates Jon’s unflappable nature and his seemingly incompetent songwriting. Jon could be the missing ingredient as they go "all the way out there," to "the furthest corners" of music, and remote Ireland, where they endlessly rehearse for an album Frank is never ready to put on tape.

‘FRANK’ Rated: R Opens Friday at Kahala 8

Jon’s in, but he’s got a lot of questions. Who IS Frank? Does he ever take the mask off? (No.) Is he mad?

"Jon, you’re just going to have to go with this," Don says. Frank, he assures Jon, is the "100 percent sanest cat I’ve ever met."

It’s where Don and Frank met that’s the key to this jarring jewel of weirdness.

The "real" Frank was this British comic named Chris Sievey, who wore an identical head for a character he played named Frank Sidebottom. Add in a band that summons up memories of Devo, The Residents, Captain Beefheart and others off the sonic beaten path, and you’ve got a story not unlike a hundred "band trying to make it" tales, with more than a whiff of insanity about it.

Gleeson, son of Irish actor Brendan and star of the warm "About Time" in his own right, is well-cast as the innocent trying to hold his own with the cranky weirdos he tossed his lot with. Gyllenhaal is on-the-nose alarming as the scary Clara.

And Fassbender, his face hidden in a mask, singing and charming when Frank isn’t off-the-rails despairing and naive even in his sanest moments, is a hidden delight. He makes Frank both a puzzle at the center of the picture and the heart of its humor. Frank has to either get his emotions across with his speaking or singing voice, or give you a hint of what his face is doing inside that mask.

"Underneath, I’m giving you a welcoming smile."

And that "welcoming smile" goes not just for Frank, the character, but this puzzling, beguiling comedy of the same name.

Review by Roger Moore, McClatchy Newspapers