Sandra Demarest arrives at the Caldwell estate, and announces to Mark Caldwell that she was secretly married to his nephew James, who recently died. Mark does not believe her, but allows her to remain at the manor while a search is made for a missing will that would prove her claim. Sandra befriends James' sister, Julie, who tells of strange noises and agonized screams from the laboratory wing of the estate. Between verbal duels with Mark, Sandra secretly investigates the lab and learns that ominous things really are happening.