After not hearing from her in two years, Lucy receives a telephone call from her only sibling, younger Martha, who eventually accepts Lucy's offer to come pick her up. Martha, who only mentions that she was living with a boyfriend in upstate New York, ends up staying with Lucy and her new husband, Ted, an architect, in their rented Connecticut lake-front recreational home. During her stay with them, Martha seems unfocused about what she will do with her life. She also displays inappropriate behavior, not matching anything resembling western convention. Lucy doesn't press Martha on the root cause of the behavior because of their family history, and Lucy's resulting want to provide unconditional support and love to her only family. Ted tolerates the behavior if only to keep Lucy happy. What Martha has not told them is that those two years she was renamed Marcy May and were spent with a cult, her behavior being what she was indoctrinated into as being what was done with that group. Situations with Lucy and Ted trigger those memories for Martha and resulting behavior. Even in her brainwashed state, Martha became to realize that certain actions by the cult were not what she knew to be right, and was able to escape from them. But as her time with Lucy and Ted progresses, Martha begins to lose track of what memories are real and what are phantom, which is exacerbated by her belief that the cult is after her. In her paranoid state, Martha becomes a risk not only to Lucy and Ted but to herself.