Jon, an aspiring songwriter portrayed by Domhnall Gleeson, encounters a man attempting to drown himself while taking a stroll on the beach in his town. This man is soon hospitalized, leading Jon to meet Don (Scoot McNairy), who reveals that the man was a keyboardist for an experimental band named the Soronprfbs, which he manages. Jon, excited about the opportunity, shares that he plays keyboards and is invited to join the band that very night.
Upon meeting the band members, Jon finds that they are skeptical of his abilities, with the exception of Frank (Michael Fassbender), the enigmatic band leader who wears a distinctive Papier-Mache mask. The concert goes off without a hitch until Clara (Maggie Gyllenhaal) breaks her theremin mid-performance and storms offstage in frustration. Despite Clara's apparent disdain, Frank offers Jon a full-time place in the band, leading him to accompany them to Ireland, where they will spend the year recording their first album in an isolated cabin.
As the recording progresses, the dynamic within the band becomes increasingly tense, especially with Clara, who mercilessly critiques Jon's perceived lack of talent. In conversations with Don, Jon learns about the harsh realities of artistic aspiration, as Don reveals he too sought to be a songwriter, only to falter along the way. He emphasizes that Frank's uniqueness is a rare quality that Jon may never truly match. This devastating realization underscores Don's own struggles with depression.
Jon clutches onto the hope that experiencing the challenges Frank faced due to his mental illness and supposed traumatic childhood will elevate his own creativity, believing that this journey with the band is what he needs. However, everything takes a dark turn when Jon discovers Frank's lifeless body hanging from a tree after a day of recording. In a shocking twist, they reveal that it was Don wearing Frank's mask all along. After Don’s cremation, Jon learns that each keyboard player for the band has grappled with their own mental crises.
Later, Jon shares that he has been uploading their recording sessions online, leading to a small following and an invitation to perform at South by Southwest. Clara, however, vehemently opposes this idea, accusing Jon of manipulating Frank and instigating grandiose fantasies about fame. In a moment of passion born from conflict, Clara and Jon share a brief encounter, but Clara harshly declares that she finds him repugnant and precludes any romantic future together. Undeterred by Clara's warnings, Jon and Frank set their sights on popularity, aiming to create "extremely likable music."
Upon their arrival in Austin, Texas, the band's ambition is met with reality when they realize they are largely unknown and the crowd lacks any familiarity with their music. This disheartening news sends Frank spiraling into panic, prompting Clara to beg Jon to convince him not to perform. Jon, however, insists on pushing forward, believing the gig is essential for their success. As tensions rise, a confrontation ensues where Clara stabs Jon before being arrested. In the aftermath, the remaining band members decide to part ways with Jon, leaving him and Frank as the only duo.
As performing beckons, Jon announces it's the "best day of his life" and starts singing one of his songs. But when Frank collapses mid-performance, Jon rushes to help, only to be met with a frantic confession from Frank about the quality of his music. An overwhelming breakdown ensues, leading to Frank evacuating the venue in sheer despair, leaving behind only the remnants of his mask.
Some time later, despite his efforts to find Frank, Jon's quest seems fruitless. Nevertheless, he finally locates Clara, Nana, and Baraque at a bar where they are now performing. Determined to track down Frank, Jon eventually finds him in his hometown of Bluff, Kansas, and learns from Frank's family that he grew up in a loving environment with no traumatic events shaping his artistry or mental health challenges. This disheartening revelation shatters Jon's previous notions about genius.
In an emotional confrontation, Jon sees Frank without his mask for the first time, only to discover a profoundly vulnerable man scarred by years of concealed identity. Jon expresses remorse for his actions and the band's disbandment, and he invites Frank to join the others at the bar. As Frank hesitantly approaches the stage, he begins to sing, slowly re-emerging from the shadows, while Jon takes a step back, allowing Frank to reclaim his place among the bandmates, leaving behind his own hopes and aspirations.
