Brandon, portrayed by Michael Fassbender, is a dashing and accomplished thirty-something businessman based in the vibrant city of New York. However, beneath his polished exterior lies a deep, hidden struggle—an all-consuming sexual addiction. His day takes an unexpected turn when he flirts with a young woman on the subway, only to discover that she wears both a wedding and engagement ring. She vanishes into the bustling crowd after they exit, leaving Brandon more intrigued than ever.
After work, Brandon joins his boss, David (played by James Badge Dale), at a club where they encounter three women. In a rather surprising twist of fate, Brandon finds himself in bed with the very woman David had his sights set on. Returning home, Brandon's night takes an even stranger turn when he believes a burglar has entered his abode, only to find his sister, Sissy (represented by Carey Mulligan), showering in his bathroom. Sissy, also grappling with the remnants of their troubled childhood—details of which unfold but never fully reveal themselves—has just moved back from Los Angeles where she struggled as a singer. Seeking refuge, she asks Brandon if she can stay with him temporarily, to which he reluctantly agrees.
The following evening, Brandon and David watch Sissy perform a soulful rendition of "New York, New York" at a jazz club. After the performance, she engages in light conversation with Brandon and David. However, when Brandon steps out to check his mail, he’s blindsided by the sound of Sissy and David in an intimate encounter in his own bedroom. Disturbed, Brandon rushes out into the city streets with Bach’s piano sonatas filling his ears, trying to escape the reality of his life. Later that night, Sissy sneaks into his bed, seeking comfort, but Brandon’s anger flares as he demands she return to the couch.
At his workplace, Brandon attempts to foster a connection with his co-worker, Marianne (played by Nicole Beharie), and invites her to dinner, which she cheerfully accepts. Their meal, while enjoyable despite the interruptions from an awkward waiter, ends without Brandon making a move, leaving him to return home lonely.
Amidst their turbulent living situation, Sissy inadvertently stumbles upon Brandon engaging in a private moment, discovering his laptop filled with explicit content. A heated argument ensues as Brandon accuses her of invading his privacy, leading him to purge his apartment of all traces of his addiction. The subsequent day, in an upscale hotel room, he again finds himself unable to engage sexually with Marianne due to unresolved issues, leaving him feeling isolated and humiliated.
Brandon's longing for connection leads him to a bar where he tries to seduce another woman, but he receives a brutal beating from her irate boyfriend. Dejected and battered, he faces further rejection at a nightclub before seeking solace in a gay bar, where he engages in anonymous encounters. Despite this, his turmoil persists, as he later receives a chilling voicemail from Sissy, prompting him to rush home amidst the unfolding chaos of a suspected suicide on the subway tracks.
Upon his return, Brandon finds Sissy critically wounded after a suicide attempt. Having survived, she ends up hospitalized, where he visits her but feels utterly broken, collapsing in the rain as he leaves.
In the film's haunting conclusion, we see Brandon back on the train, his routine unchanged, yet he encounters the same woman from earlier, her engagement ring glinting as she smiles at him. As anticipation builds, the screen fades to black, leaving the audience questioning whether he will choose to pursue her like before, or finally confront the shadows of his own addiction.
