Successful and handsome New Yorker Brandon (Michael Fassbender) seems to live an ordinary life, but he hides a terrible secret behind his mask of normalcy: Brandon is a sex addict. His constant need for gratification numbs him to just about everything else. But, when Sissy (Carey Mulligan), Brandon's needy sister, unexpectedly blows into town, crashes at his apartment and invades his privacy, Brandon is finally forced to confront his addiction head-on.