One evening the Collins find their maid Mrs. Murdock at the end of their steps dead, neck broken. Obviously she had an accident. Consequently they need a new babysitter for an upcoming big party. The sensible Helen meets little Susan Collins at the discount market and likes her, so she offers to do the job. She doesn't know Susan's teenage brother Mark - technically skilled and good in school, but restive and cunning. As soon as the parents have left, he starts psychologically terrorizing Helen and his sister with electronic tricks.