A man takes his young wife to her family home just as a series of strange murders start.

Susannah Kelton returns with her husband, Mike, to her childhood home on an isolated island off the New England coast to inspect an old mill she has inherited. She has not been there since she was sent to New York City after both her parents died. The islanders, openly hostile to the couple, are aghast at their plans to use the millhouse, which has an ominous history, as a summer house. Mike and Susannah ignore the warnings of her brutish cousin Ethan and her morose Aunt Agatha. Curious about the shuttered room on the top floor of the millhouse, Susannah conducts an investigation. She does not know that Mike has been waylaid and beaten up by Ethan's gang of wastrels. Frightened by the dark old building, Susannah is about to leave when she is suddenly confronted and nearly assaulted by the lecherous Ethan. She takes refuge in the shuttered room, but Ethan follows her with a lighted torch. As he pursues Susannah, a strange figure lunges at him from out of the shadows, causing him to fall to his death. Mike, meanwhile, has learned the truth about the millhouse from Aunt Agatha, whose silence has been broken by the murder of Emma, Ethan's sluttish girl friend. The strange figure in the shuttered room is Sarah, Susannah's insane and physically malformed sister, who, chained in the room, has been sustained through the years by the devoted Agatha. Racing back to the millhouse, Mike arrives in time to drag Susannah from a fire started by the lighted torch Ethan dropped when he fell. Agatha then appears and locks herself in the shuttered room, choosing to perish in the flames with Susannah's terrified and bewildered sister.