When the family of recently released multiple murderer, Susan Keller, start being killed off, she is placed under protective police surveillance in a remote cottage provided by a mystery benefactor. During the night a black-clad figure creeps into the cottage and is tackled to the ground by police. The intruder (Michelle Gomez) is revealed to be Harriet Bailes, Susan's former prison governess, lover and now benefactor. Evidently devoted to Susan, Harriet is not above suspicion as the murders stack up.
