There's a moment in Frank when one character turns to another and says "You're just going to have to go with this one." And in many ways he's talking to the audience, telling us not to stress about the fact that the film’s title character permanently wears an oversized papier mache head, and to just go with it.

And Brits of a certain age won't have to suspend disbelief too much as the film is (vaguely) inspired by real-life comic character Frank Sidebottom. The onstage persona of cult comedian and musician Chris Sievey, the character popped up on TV throughout the 1980s and '90s in a similarly over-sized head, and journalist Jon Ronson played keyboards in his band for a period.

That experience prompted Ronson to write a newspaper article on the subject, and to now co-author this film, which isn't based on real events, but does revolve around a character who looks very much like Mr. Sidebottom.

This Frank is the lead singer of The Soronprfbs, a band so avant-garde that they can't even pronounce their own name. Musical outsiders yet to have a hit, they are touring the UK when proceedings commence, only for their keyboardist to have something of a meltdown, throwing himself into the sea and having to have his stomach pumped.

Step up Jon, a music-mad youngster who happens to be in the right place at the right time with the right talent – namely the ability to play the notes C, F and G – to join the band.

The adventure is cut short when their first gig together ends in disaster, but when the previous keys man is sectioned, Jon finds himself rejoining the band, largely because Frank feels he brings something 'cherishable' to the line-up.

Jon is soon holed up in a countryside cabin with the band as they endeavor to write an album, and while we don't get a glimpse at what Frank looks like underneath the head – with band-mates claiming he hasn't taken it off for 15 years – we do start to understand the strangeness bubbling beneath that bizarre surface.

Fragile, anxious, angry and sweet, Frank is a tortured musical genius trying to unlock sounds in the furtherst corners, like some maniacal cross between Captain Beefheart, Brian Wilson and Daniel Johnston. Designing his own instruments, he finds inspiration in everything around him, and in turn inspires everyone he comes into contact with.

Not enough to complete their album however, but that might be as much to do with Jon's presence as it is the group's collective writer's block. Putting the alt into alternative, aside from their eccentric frontman, The Soronprfbs are made up of a laconic manic depressive, a girl who doesn't speak, a bloke who only doesn't speak English, and a woman who seems to exist in a permanent state of rage. Onstage they look like the house band in a David Lynch movie.

But the music is good, and that twisted dynamic works. Until Jon turns up, the wide-eyed innocent initially upsetting the musical apple cart thanks to his complete lack of talent. But soon ambition gets the better of him, and seeing this as his one shot at glory, Jon’s soon driving the band towards the mainstream, even if that means selling out in the worst way possible.

What follows is a tale of both comedy and tragedy, as the group experiences the highs and lows that the music industry has to offer, including a disastrous sojourn to the U.S. that has devastating consequences for each member.

It's also a film that examines the creative process, questioning whether artistic integrity can go hand-in-hand with commercial success, and asking how artists can stay true to themselves in an industry that's come to be dominated by the marketing men.

Rather unexpectedly, Frank also has something to say on the subject of mental illness, with Michael Fassbender delivering a barnstorming performance as the likeable but deeply troubled title character. The actor has used his physicality to great effect in films like Hunger and Prometheus, and here he brilliantly utilises his body to convey the pain and anguish underneath Frank’s unexpressive exterior.

It's not all misery however, as the film is consistently funny throughout, with Domnhall Gleeson hilarious as the ordinary Jon thrown into extraordinary circumstances and making the wrong decision pretty much every step of the way.

The scenes he shares with Maggie Gyllenhaal's character – whose hatred for Jon seemingly knows no bounds – are some of the film's funniest, while his narration of events is filled with clever observations and witty one-liners (although the use of onscreen Tweets quickly wears thin).

Such juxtaposition makes for a film filled with a complex jumble of emotions, but mercifully director Lenny Abrahamson – whose last feature was the criminally underseen What Richard Did – manages to maintain a consistent tone throughout.

The Verdict

Abrahamson has therefore achieved the impressive feat of making a marvelous movie that's about music and about being in a band, while at the same time crafting an affectionate character study of a bloke in a massive papier mache head. If you're willing to go with that, you'll find Frank to be a richly rewarding film, and the title character to be one you can't help but fall in love with.