Ellen Hardy Stella Stevens works as a live-in assistant to wealthy widow Mrs. Armstrong Shelley Winters. She receives a call from the mental institution where her younger brother George Michael Burns and sister Mandy Barbara Sammeth have been living since they were accused of killing their parents when they were six and four. George is turning 18, and rather than sending him and Mandy to an adult facility, Ellen brings them back to live with her in Mrs. Armstrong’s large house. She keeps their dangerous history hidden, striving to protect them from a judgmental world while masking the truth herself.
Upon arriving at the house, Mandy insists on a designated “mad room,” a private space they can retreat to when they feel overwhelmed. Ellen reluctantly agrees to give them access to the former Mr. Armstrong’s study, even though Mrs. Armstrong refuses to let anyone in. One night, Mrs. Armstrong discovers Mandy in the study and confronts Ellen about her mounting suspicion that they are keeping something from her. As Mandy and George listen from outside, Ellen finally breaks down and reveals the grim childhood shared by the siblings and the fear that either George, Mandy, or both had killed their parents with a butcher knife. Mrs. Armstrong explains that she can’t keep the children at the house any longer and goes to bed, shaken by what she has learned.
The following morning, Ellen screams when she finds Mrs. Armstrong dead in the “mad room,” slashed by a saber. Mandy and George rush to the scene, both crying out and accusing each other of murder. Ellen quickly shifts into cover-up mode, insisting to the staff that Mrs. Armstrong has gone away on business unexpectedly. After another troubling incident at the house, George and Mandy grow uneasy with Ellen’s consistent deception and start to suspect that she may have killed Mrs. Armstrong as well as their parents.
Ellen’s nerves begin to fray, and she tries to convince her fiancé Sam [Skip Ward], the stepson of Mrs. Armstrong, that they should send the children away. Her grip finally snaps when the family dog uncovers a dismembered portion of Mrs. Armstrong’s body (her hand) and she kills the dog in a moment of desperate fear. In the final scene, Sam discovers Ellen kneeling in the basement by the furnace, a haunting echo of the night she murdered her own parents.
