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Growing up in a shitbird coastal town, wannabe keyboard player/songwriter Jon Burroughs (Domhnall Gleeson) finds his life taking off when touring avant-garde rock band Soronprfbs ask him to fill in at a gig (“Can you play C, F and G? You’re in!”). Next thing he knows, he’s off to Ireland to record an album, struggling to find his place in a makeshift family unit that includes manager Don (Scoot McNairy), who has a fetish for mannequins, and sour-faced theremin player Clara (Maggie Gyllenhaal, terrific).

But the real draw is frontman Frank (Michael Fassbender), a rambling, ranting genius/fool who invents his own musical scale and insists the band make their own instruments. Oh, and one more thing: he wears a huge papier-mâché head, on stage and off…

Based, loosely, on screenwriter Jon Ronson’s own late-’80s stint in a band led by the late Mancunian musician/comic Chris Sievey under the stage name of Frank Sidebottom, Frank steers the narrative into areas beyond biography. It’s a contentious decision, brave and bizarre, but one that Sievey would have no doubt appreciated. The important thing here is that Lenny Abrahamson’s ( Adam & Paul , What Richard Did ) movie is faithful in spirit, and for that we can rejoice.

Frank is a funny film, the band’s 11-month(!) recording stint in Ireland proving a creative maelstrom that’s all maelstrom and little creativity. Through it all, Jon blogs and tweets inanely jocular banalities to an ever-growing following while Frank never removes his head (not even in the shower) and instead vocalises his emotions and facial expressions to the nonplussed band: “Big, non-threatening grin”; “Lips pushed together, as if to say ‘Enough frivolity’.”

Not that explanation is needed, for Fassbender’s faceless performance offers a masterclass in body language, and that wide-eyed, open-mouthed mask invites band members, and viewers, to project their own readings.

The third act is more conventional, with a chaotic road trip to America proving neither as interesting nor as compelling as the spiky material that came before. But Ronson and Abrahamson keep their heads to offer a perfectly judged, moving finale, and they have plenty to say about public image and the popular myth of creative genius being born of personal torment.

You’ll want Frank in your life – as the cryptic vocalist says of Jon: “He brings something cherishable.”

Verdict: A glorious curveball: surreal, abstract, laugh-out-loud funny and quietly moving. And the ‘tunes’ – performed live by the cast and sung by Fassbender – are a knockout.