Brandon Sullivan is a self-loathing sex-addict. After having a fight with his emotionally needy/unstable sister Sissy, Brandon goes out on a sex "binge" engaging in various harmful acts including random gay sex, purposefully hitting on another man's girlfriend so he will get beaten, and engaging in an anonymous threesome. He returns home to find Sissy has slit her wrists, although her life is saved at the hospital. Brandon finally appears to be truly remorseful about his lifestyle and treatment of his sister, but we are left unsure about whether he can/will actually resist his urges.
Brandon Sullivan (Michael Fassbender) is a successful account manager at a huge firm, and a sex addict (and, a caffeine addict as well). He attempts to seduce a married woman that he sees on a subway, pays for prostitutes, constantly watches porn online at home and work, and has sex with women in back alleys. He comes home one day to find his sister Sissy (Carey Mulligan) has temporarily moved in. It becomes quickly apparent that the two have an unhealthy relationship. She is emotionally needy - constantly telling every man she meets that she loves them. Brandon is clearly uncomfortable with his sister, and becomes more stressed out when Sissy not only sleeps with his married boss, but constantly calls his boss afterwards. She also climbs into his bed at night, asking to be hugged, and hangs out with him in various forms of undress. Sissy is a very talented singer, who brings Brandon to tears with her voice, although he will never praise her talent.
Things soon begin to unravel for Brandon. His advances on women become less suave and more desparate. Although his employer discovers that his hard drive at work is chock-full of porn, it is blamed on a virus or prank. He goes on several dates with a co-worker named Marianne (Nicole Beharie), but when they begin to get intimate, Brandon is unable to perform. In a fit of self-loathing rage, he kicks Marianne out of his hotel room and hires a prostitute to engage in rough sex with him. Brandon also becomes embarassed when Sissy discovers his pronographic materials. As a resuly, Brandon throws away his sizable stash of pornography as well as his personal computer.
Unfortunately, Brandon relapses. He fights with Sissy after he physically (not sexually) assaults her and Sissy states that if she left him alone, she would never hear from him again. After the altercation, Brandon goes out into the night, ignoring Sissy's phone calls and pleas to reaffirm that she is not a bad person. He knowingly attempts to pick-up a girl in a bar in front of her boyfriend using incredibly lurid statements, resulting in him getting beaten to a pulp. He is rejected from entering a night club, so he instead enters a gay hook-up club where he makes out with and receives fellatio from an unidentified man. He then goes to a stranger's apartment where he engages in a torrid threesome with the two female strangers he meets there. Clearly, Brandon has fallen off the rails. As he makes his way home in a hungover-type state, he realizes that Sissy is not answering his calls. When he gets to his apartment, he finds Sissy has slit her wrists and is unconscious.
Brandon is mortified and calls an ambulance. He shows true remorse and stays by her bedside until she recovers. It appears that Brandon is actually trying to change his ways and bring his addiction under control despite his disastrous attempt to do so the first time. The film ends with Brandon on the subway where he sees the married woman he attempted to seduce at the beginning. She recognizes Brandon and purposefully flirts with him, and the movie ends without us knowing whether Brandon will maintain his resolve, or succumb to his destructive nature.