The three surviving children of Titus W. Merrye, who represent the end of his family's line, live in a dilapidated mansion where patient servant Bruno (Lon Chaney, Jr.) watches over the increasingly eccentric Virginia (Jill Banner), Ralph (Sid Haig), and Elizabeth (Beverly Washburn). All three Merrye siblings suffer from the same rare disease that felled their father and the other members of his family -- "Merrye Syndrome," a neurological ailment that begins to manifest itself at the age of ten, causing the brain to slowly decay and sending its victims into an alternately violent and infantile state. Bald, inarticulate Ralph is supposed to be a vegetarian, but "can eat anything he can catch," while Virginia, who seems to be in a perpetual dream state, imagines herself as a human spider and catches people in her "web" (a large net) and then kills them. While it might seem best to let nature to take its course and allow the family's sad legacy to die out, the Merrye siblings have two distant cousins, Emily Howe (Carol Ohmart) and Peter Howe (Quinn K. Redeker), who are interested in laying claim to the family mansion and any money remaining in the Merrye Estate. But not long after they pay a visit to Bruno, they start to have serious regrets about their decision to see the family.