Ellen Hardy, working as a live-in assistant to wealthy widow Mrs. Armstrong, gets a call from the mental institution where her younger brother George and sister Mandy have been living since they were accused of killing their parents when they were six and four years old. George is turning 18, and rather than send him and Mandy to an adult facility, Ellen takes them back to live with her in Mrs. Armstrong’s large house. Afraid of what their reception would be if the others knew the truth, Ellen conceals their dangerous history.
Upon arriving at the house, Mandy insists that she and George have a designated “mad room,” a place where they can go to be alone when they are frustrated. Ellen reluctantly agrees to give them access to the former Mr. Armstrong’s study despite the fact that Mrs. Armstrong refuses to let anyone in it. One night Mrs. Armstrong discovers Mandy in the study, and confronts Ellen about her mounting suspicion that they are keeping something from her. While Mandy and George eavesdrop from outside the room, Ellen finally breaks down under her questioning and tells Mrs. Armstrong about their grim childhood and the suspicion that either George, Mandy, or the both of them had killed their parents with a butcher knife. Mrs. Armstrong explains to Ellen that she can’t keep the children at the house anymore, and goes to bed frightened.
The following morning, Ellen screams when she discovers Mrs. Armstrong dead in the “mad room,” slashed by a saber. Mandy and George run to the scene, and both scream in horror and accuse each other of being the murderer. Ellen quickly turns into action, and to cover-up the crime, insists that they tell the rest of the staff that Mrs. Armstrong has gone away on business unexpectedly. After another incident at the house, George and Mandy begin to find Ellen’s ability to lie unsettling and suspect her of killing Mrs. Armstrong as well as their parents.
Ellen’s nerves begin to fray, and tries to convince her fiance Sam, the stepson of Mrs. Armstrong, that they should send the children away. She finally snaps after seeing that the family dog has discovered the dismembered body of Mrs. Armstrong and has brought a single hand out into the yard. She kills the dog and in the final scene is discovered by Sam, kneeling in the basement by the furnace as she once did the night she murdered her parents.