Michael Fassbender’s eccentric avant-garde rock singer Frank always wears a giant papier-mâché head.

Offbeat doesn’t begin to describe this oddball comedy in which Domhnall Gleeson’s callow wannabe musician is recruited into Frank’s band as its new keyboard player.

He joins this bunch of misfits, including Maggie Gyllenhaal’s prickly Theremin player, for a year of recording sessions in a remote log cabin, followed by a disastrous trip to a US festival, during which he strives to uncover the real Frank and learn the secret of his musical genius.

Very loosely inspired by cult showbiz character Frank Sidebottom, this is a drolly funny, surprisingly touching portrait of a charismatic musical outsider. And Fassbender is brilliant beneath that absurd head, conveying all you need to know about Frank’s joys and woes and troubled soul by his body language alone.