Jon (Domhnall Gleeson) is a young man with a dream: He wants to be in a band. He wanders the streets looking for something, anything, to give him some inspiration to write a song, and spends endless hours twinkling at his keyboard. Yet everything he pens is absolute crap and he seems much better at writing tweets than lyrics.
When Jon gets a sudden offer to fill in for a keyboard player in a cult band from out of town, he jumps at the chance. There he meets Frank (Michael Fassbender), the band's eccentric singer — best described as "random," in the parlance of our times — who not only performs wearing an oversized papier-mache puppet head, but wears it 24/7 and never breaks character.
Frank is at once childlike, bizarre and on the edge, but more important to Jon, it seems, is that he's a gigging musician. When Frank's manager (Scoot McNairy) asks Jon to join Frank's band (Maggie Gyllenhaal, Carla Azar and Francois Civil) for a recording session in a remote Irish cottage, he jumps at the chance, though he's shocked when he arrives to find rationed food and a cultlike process of deconditioning.