Chris Sievey is a curious footnote in the history of alternative music and performance art. He was an animator, comedian and jobbing musician with various bands and appeared on a number of TV shows, mostly in the North West, during the ’80s and ’90s.

So far, so unremarkable. Yet as Frank Sidebottom and wearing a papier maché head, Sievey became something of an iconic and mysterious figure.

Among his colleagues in one of his acts, the Oh Blimey Big Band, were DJ Mark Radcliffe and, pertinent here, Jon Ronson, the journalist who has already written about his time playing keyboards in the 1980s.

Ronson has penned a new version of the big-headed figure although Sievey, who died in 2010, resisted biopic in favour of a fictional tale to maintain the enigma.

Perhaps that is the first clue to the ultimate failure of this film.

Ronson uses something of his own story – plucked from obscurity to join a band – as a way into Frank. Suburban Jon (Domhnall Gleeson) is co-opted into a mad collective of musicians by chance and is taken to Ireland for a year of musical experimentation, drugs and soul-searching.

Don (Scoot McNairy) is manager and among the snobbish musos are fierce and protective Clara, an underused Maggie Gyllenhaal, who takes against Jon and sees him as the possible agent of their destruction. Jon wants to be famous, Clara refuses to yield to populism and prefers their impenetrably bonkers musical seclusion.

And at the centre of this drop-out collective is Frank (cleverly realised by Michael Fassbender) always in the mask, a guru, a genius, an eccentric. Their year-long investigation of song and self is the most effective part of the film, full of comedy and energetic riffs. Presumably, this is the section closest to biography.

Then the baton is passed to fiction and the fiction fumbles the handover.

When they leave Ireland to play at the SXSW festival, the film abandons its chaotic, indulgent free form and attempts, instead to become something altogether different. A Proper Film.

Frank stops being the seer of all things and becomes a jibbering, weepy headbanger for reasons that are not entirely plausible except within the confines of a Hollywood pitch meeting where arc, plot, peril and closure are prerequisites.

As Frank falls apart, so does the film. We are asked to ignore all that went before and start again with this new enfeebled, clueless Frank. Jon suddenly has a “mission” to mend his bumbling mentor and the whole final act, and therefore the whole film, relinquishes its claim for originality – and for our attention.

The final, cowardly capitulation is that we get to see Frank’s face and the entire argument about identity and psyche is tossed away for the price of a momentary thrill.

A good idea descends into a messy, empty-hearted compromise which is a waste of Fassbender, Gyllenhaal, Gleeson and that early whimsical investigation into the artistic process.

Frank
(15) 95mins
★★✩✩✩