Ellen Hardy is the companion to Mrs. Armstrong, a wealthy widow who lives with her stepson, Sam Aller, and her maid-secretary, Chris, in a huge old mansion. Ellen and Sam are preparing to marry when Ellen learns that her teenage brother and sister, George and Mandy, are to be released from the mental institution where they have been confined; as young children, they were the prime suspects in the brutal knife slaying of their sleeping parents, but it was never determined which of the two was responsible. Claiming that the youngsters were staying with an uncle who recently died, Ellen persuades the reluctant Mrs. Armstrong to allow them to move into the mansion. George and Mandy tell Ellen that they need a "mad room" where they can rid themselves of emotional hostilities, and Ellen secretly lets them use an attic study which Mrs. Armstrong has kept locked since her husband's death. Eventually, however, the widow learns about both the "mad room" and the children's past and demands that they leave. That night, Ellen finds Mrs. Armstrong hacked to death with one of her late husband's sabers. The children first suspect each other, but they finally realize that their sister, in her misguided attempts to shield them, committed both the present and the past murders. To cover up the crime, Ellen disposes of Mrs. Armstrong's corpse in a river and plants evidence indicating that the death was accidental. Unbeknown to Ellen, however, the family dog has carried off one of Mrs. Armstrong's severed hands. Ellen's mind totally snaps when she sees the animal carrying the hand, and Sam catches her as she is butchering the dog in the mansion basement.