Simon Dillon reviews the offbeat black comedy

As far as I understand it, Frank is a fictional version of a book by a journalist who played keyboards in the Frank Sidebottom Oh Blimey Big Band. In the band, Frank Sidebottom wore a big fake head and nobody except his fellow band members knew his true identity.

The keyboard player here is Jon (Domhnall Gleeson), a wannabe songwriter who finds himself in trouble when he blags his way into an eccentric fringe pop group fronted by the enigmatic and always masked Frank (Michael Fassbender). The band go to Ireland to record a new album, and Jon is eager to prove himself to his new band mates. Rivalries, jealousies, artistic hissy fits and all manner of surreal craziness ensues, culminating in a much darker turn of events.

Although director Lenny Abrahamson and screenwriters Jon Ronson (who wrote the book) and Peter Straughan keep the offbeat black comedy to the fore, this is ultimately a much darker, much more heartfelt, much more poignant film than it first appears to be. This is not a coming of age success story but instead a serious look at the dynamics of creativity and mental illness. Many clichéd assumptions are challenged - for example, the notion that to be an artist you need to have suffered, and the idea that the mentally ill invariably come from horrendously abusive backgrounds. No easy answers are given, and nothing about the film's devastating third act feels phoney or forced. Indeed, the film's ideas, especially about mental illness, ultimately have a real ring of truth.

Of course, for this to work a first rate cast is needed, and Fassbender is amazing in the lead role. Somehow he manages to get across the essence of his character's thoughts and feelings without using his face - a truly remarkable feat. Fassbender is ably supported by the rest of the cast, especially Gleeson and Maggie Gyllenhaal's borderline psychopathic theremin player.

Frank is not for everyone. Some will find it too bizarre to engage with. Others will be turned off by the swearing. It is, nevertheless, a very fine piece of work well worth checking out for those with the temperament for it.