A young woman named Martha escapes from an abusive cult in the Catskill Mountains, led by a charismatic but manipulative leader named Patrick. After fleeing, she reaches out to her estranged older sister Lucy, whom she hasn't seen in years, and Lucy's new husband, Ted. They take Martha in, unaware of the specifics of her recent past, trying to reintegrate her into a normal life in their lakeside vacation home. The film explores Martha's struggle to assimilate into everyday society, as she experiences increasing paranoia and disorientation. Martha is haunted by painful memories and her profound sense of displacement; the narrative unfolds through a series of flashbacks, depicting Martha's life within the commune, the disturbing dynamics and rituals she was subjected to, and the familial bonds she formed therein. The juxtaposition between Martha's past and present blurs as she attempts to reconcile her cult identity, Marcy May, with her familial one, Martha.

While Martha's sister and brother-in-law exhibit patience and concern, they are also confused and frustrated by her erratic behavior, illustrating the complexities of post-trauma recovery and the effects of prolonged psychological manipulation. Martha is stuck in a perpetual state of fear and confusion, finding it difficult to distinguish reality from her indoctrinated past. Her experiences at the commune begin to encroach on her present life, leading to a growing tension within the household and raising questions about Martha's ability to undo her deeply ingrained conditioning. The film captures the psychological landscape of a cult escapee, exposing the profound impact such an environment has on an individual's sense of identity and reality.