Recently released from a sanatorium, Kate Hersey is seemingly unaware that her husband, John, has been pursuing an affair with her sister, Marianne. When a series of strange events begin to plague the block of flats in which (in John's absence) the sisters live together, Marianne assumes someone is confusing her with Kate. But Kate, aware of her sister's betrayal, has her own plan to gain both her husband's money and revenge, before fleeing with her own lover.
