The story revolves around Martha (Elizabeth Olsen) who lives in an abusive and totalitarian cult in the Catskill Mountains. The cult is lead by the charismatic but deranged Patrick who takes an intense interest in Martha. After escaping from the cult Martha goes to her friend and sister Lucy and tells her that she needs help and a place to stay. Lucy acquiesces to the request and brings Martha to stay with her and her husband Ted (Hugh Dancy).
During the following weeks after moving in Martha exhibits rather unusual behavior and does not tell them that she had been a member of the cult, only saying she was staying with a boyfriend. So peculiar is her behavior that she walks in on her sister and Ted having sex and promptly gets into bed with them. Her sister and husband are horrified and tell her that there is something decidedly wrong with her. What follows is a psychological descent of the grimmest proportions.