At the decaying Merrye House, locals whisper about a place that seems to breathe its own gloom. The three Merrye children—Ralph, Virginia, and Elizabeth—have lived in near total isolation with Bruno, their family's devoted chauffeur, ever since their parents died. Ralph, Sid Haig, is a sexually confident yet mentally simple young man who moves through the house via the dumb-waiter. Virginia, Jill Banner, is obsessed with spiders, and has already murdered visitors in a morbid game of “spider,” rigging a window to snap shut on them before using butcher knives to finish the job. Elizabeth, Beverly Washburn, appears calm and responsible to outsiders, but underneath is a calculating side that is almost consumed by hate. The trio’s bond with Bruno, their loyal chauffeur, remains strong, and they quietly rely on him even as his warnings about their impulses grow harder to ignore.
Into this brittle balance comes news that Peter Howe, his sister Emily, their lawyer Schlocker, and his secretary Ann are traveling to claim the Merrye property. A delivery man, Mantan Moreland, serves notice that distant relatives intend to evaluate the estate and the peculiar children who live there. Bruno quickly tries to stage a veneer of normalcy, coaching the children in social etiquette so they can pass for ordinary guests in the eyes of outsiders. Schlocker, visibly upset by Bruno’s guardianship and the absence of formal schooling for the children, insists that the family’s situation be clarified and that the children be treated as wards of the state if necessary. He makes it clear that he and Emily intend to spend the night to assess the situation, while Peter, overwhelmed by the mansion’s uncanny aura, takes Ann into town since there are only two rooms available.
The visit unsettles everyone. Schlocker’s curiosity leads him to explore the basement, where he discovers a pit containing the Merrye patriarch’s hidden siblings. Virginia and Elizabeth act swiftly, murdering Schlocker to prevent his report from reaching the outside world. Bruno, sensing that a cover-up has become impossible, leaves to fetch dynamite from a nearby construction site, intending to blow up the entire house rather than allow the children to be institutionalized or betrayed. In his absence, Emily finds Schlocker’s body as Virginia and Elizabeth scramble to dispose of it. The danger intensifies when the girls chase Emily into the woods, where she is captured and sexually assaulted by Ralph, a brutal moment that shadows the family’s already dark dynamics.
With no rooms available in town, Peter and Ann return to the mansion. Elizabeth, worried about potential exposure, escorts Ann to her room and allows Ralph to close in on her, while Virginia loses herself in a cruel game of “spider,” tying Peter to a chair and preparing to strike. Elizabeth calls for help with Ann, who struggles against her captors and against Ralph. In the woods, Emily awakens traumatized and delirious, and she makes her way back to the house, where she confronts Ralph and his sisters in a clash that underscores the volatile power dynamics at play.
Peter manages to escape confinement and frees Ann, and Bruno arrives with dynamite, urging Peter to flee. Peter escorts Ann to safety as the Merrye home buckles under the mounting violence, and the house finally explodes behind them, taking Bruno and the Merrye siblings with it. Peter, now the sole heir, inherits the vast Merrye fortune, marries Ann, and writes a book documenting the mystery of the Merrye Syndrome. His branch of the family has never been affected by the condition, a quiet note of irony in the wake of the carnage. Ten years later, as Peter and Ann’s daughter wanders outside, she becomes fascinated by a spider, a chilling reminder of the era-spanning shadow of the Merrye lineage.
