At the decaying Merrye House, feared by locals, the Merrye children Ralph, Virginia, and Elizabeth have lived in seclusion with the family
chauffeur Bruno ever since their parents died. All three have advanced Merrye Syndrome, a genetic affliction unique to members of the family which causes them, starting in late childhood, to
regress down the evolutionary ladder mentally and physically. They exhibit a playful innocence well before their years, mixed with feral madness. Ralph is a sexually advanced, but mentally deficient
simpleton who moves through the house via the
dumb-waiter. Virginia is obsessed with spiders, and has more than once murdered visitors in a game of "spider", trapping them by rigging a window to snap shut on them, before hacking them to death with butcher knives. Elizabeth is regarded by Bruno as the most responsible of the three, but is also conniving and infatuated with the concept of hate. The children hold a steadfast respect and affection for Bruno, but increasingly disregard his admonitions against their darker
impulses. Virginia's latest victim is a delivery man serving notice that Peter Howe and his sister Emily, distant relatives of the Merrye family, are coming with their lawyer Schlocker and his secretary Ann, seeking to claim the property as rightful heirs. Bruno hastily coaches the children in enough social etiquette to pass muster before the visitors arrive. Schlocker is outraged that the children have been in the sole care of Bruno (who did not acquire legal guardianship) and have never attended school. Bruno tells him the children are
intellectually disabled and resists the suggestion to put them in an institution, having sworn to their father to protect them for life. Emily and Schlocker insist on staying overnight to examine the situation, but with only two rooms available, Peter takes Ann into town to stay at an inn. Schlocker investigates the house, going down to the basement, where he finds the Merrye patriarch's siblings are kept in a pit. Virginia and Elizabeth murder Schlocker to prevent him from reporting this discovery. Bruno realizes it will be impossible to keep this latest murder covered up, and leaves to fetch dynamite from a nearby construction site, planning to blow up the entire Merrye family rather than allow them to be confined. In his absence, Emily finds Schlocker's body as Virginia and Elizabeth are trying to dispose of it. The girls chase Emily out into the woods, where she is captured and raped by Ralph. Finding no rooms available in town, Peter and Ann return to the mansion. Elizabeth, fearing the consequences if their crimes are discovered, escorts Ann to her room, leading her into the clutches of Ralph, while Virginia starts a game of "spider" with Peter, tying him to a chair and preparing to "sting" him with her knives. Elizabeth intervenes to ask for help with Ann, who is struggling against her and Ralph. In the woods, Emily awakes traumatised and delirious. Sexually aggressive and murderous, she returns to the house and attacks Ralph while his sisters defend him. Meanwhile, Peter escapes his confinement and frees Ann. Bruno arrives with the dynamite and urges Peter to flee. Peter escorts Ann to safety as the house explodes behind them, killing Bruno and the Merrye family. Peter, as the sole remaining heir, inherits the vast Merrye family fortune, marries Ann, and writes a book on the Merrye Syndrome phenomenon. His branch of the family, being rather distant, has never been afflicted by the syndrome. However, ten years later, while wandering outside, Peter and Ann's young daughter is fascinated by a spider. ==Cast==