In a small island off the American coast, the Whateleys live in an old mill where a mysterious bloody being creates an atmosphere of horror. After her parents get killed by lightning, young Susannah is sent to New York by her aunt Agatha, who wants her to avoid the family curse. Years later, Susannah Whately Kelton (Carol Lynley), now married, persuades her husband to spend a vacation in the abandoned mill. Once on the island, Susannah and Mike Kelton (Gig Young) soon find themselves exposed to the hostility of a gang of thugs led by Ethan (Oliver Reed), Susannah's brutal cousin.