Following the suicide of the Soronprfbs manager Don (Scoot McNairy), the band's new keyboardist Jon (Domhnall Gleeson) steadily becomes more controlling of the band and even gets them a performance at SXSW.
Prior to the performance, Clara (Maggie Gyllenhaal) angrily stabs Jon in the leg, resulting in her being arrested. The other two band members, who do not like Jon's influence on Frank (Michael Fassbender), proceed to quit, leaving Jon and Frank to perform as a duo. As Jon begins playing a song on stage, Frank faints from stage fright, ruining the performance.
The next day, Jon angrily tells Frank to take off his large papier-mâché head and the two get into a fight, which ends with Frank getting hit by a car, which destroys the head, though Frank manages to run away. Jon is also run over by a car and spends some time recuperating in a hospital, before he goes on a search for Frank.
Jon eventually finds Frank living with his parents, who explain to Jon that Frank is a mentally ill person, who has been wearing fake heads since he was 14 years old. The film ends with Frank showing up at a performance by the Soronprfbs, who are now fronted by Clara. Frank begins reciting what sounds like gibberish, but after being given the mic by Clara, it turns into the heavily emotional song "I Love You All," which the band performs while Jon watches from the bar.
Thanks Sean K!