Director: Lenny Abrahamson
Cast: Michael Fassbender, Domhnall Gleeson, Maggie Gyllenhaal
Certificate: 15

We all know the story. Guy joins unknown band. Guy discovers his inner creative self. Unknown band becomes famous.

Frank is not that story.

Inspired by the persona of Frank Sidebottom, Lenny Abrahamson's film isn't a straight-out biopic. It isn't a comedy either. Or a drama. Or a musical. It's a mix of all three - and, as a result, manages that to be that rare thing: unpredictable.

It's written by The Men Who Stare at Goats author Jon Ronson, who was once a keyboardist in Sidebottom's band. Effectively played by Domnhall Gleeson, events unfold from his perspective, but he's not the main character: he's the conduit through which we observe the titular Frank, a man with a giant papier mâché head.

Jon winds up tinkling the ivories by chance, after the previous piano player tries to kill himself - a worrying band tradition. That dark current of suicide and mental illness reverberates through the film, a serious pedal note underneath the cheerful main tune. No one in the group (whose name is, of course, impossible to pronounce) seems to be all there, from melancholic manager Don (an unrecognisable Scott McNairy) to the perpetually angry Clara (a hilarious Maggie Gyllenhaal), who plays the theremin with eye-bulging intensity.

Ronson's script modulates between the happy and sad, never signalling the key changes - even when Frank starts telling Jon what his facial expressions are beneath his massive, blank mask. "Encouraging smile," he says, his gaping eyes staring coldly ahead.

The perfect equilibrium between downbeat and daft is found, naturally, in the music: when the band play, Michael Fassbender leaps about inside Frank's head - an actor more liberated, you sense, than limited - while shouting nonsensical phrases with glee. Clara looks on possessively. Jon just looks confused.

Frank writes songs about anything, including lone-standing tufts of carpet, a knack that Jon tries to emulate - poorly. The whole group are sucked in by Sidebottom's whirlwind of ideas, inspired and intimidated in equal measure. But this isn't the story you know: where you might expect Jon to learn something about himself or grow as a musician, if anything, the opposite seems to happen - he mostly ruins everything. All the while, he posts videos on social media to an audience of what Frank hopes will be loving fans.

The chorus of commercial vs musical success and artistic vs mental imbalance results in an ensemble piece that manages to be something delightfully unique. This isn't a biopic. Rather, it's a tribute to creativity in its purest form - original and unpredictable. It just so happens that this form has a name. And wears a giant papier mâché head.