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.

Mark tiene 15 años y no es como los demás chicos de su edad. Su carácter retorcido y endiablado ha conseguido alterar la vida de su familia y sus compañeros de escuela. ¿Es Mark sólo un chico "malo" o tiene verdaderos problemas mentales? La respuesta queda clara cuando una loca carrera por la venganza y el poder se convierte en una trampa mortal.