Dr. Roger Girard (Bruce Dern) is a rich scientist experimenting with head transplantation. His caretaker has a son, Danny (John Bloom), who is an extremely strong full-grown man, but he has the mind of a child due to brain damage sustained in a mine accident. In an unusual turn of events, Manuel Cass (Albert Cole), a recently escaped mental patient and serial killer, has murdered Dr. Girard's caretaker and is seriously injured himself. Given an unprecedented chance to use human subjects -- a mortally wounded psychotic and a disabled man with little chance of surviving on his own, neither of whom he thinks will be missed -- Dr. Girard transplants Cass's head onto Danny's body to prove that his techniques can be applied to human beings. The new creature, with one head of a murderer and the other with the mental capacity of an eight-year old attached to an extremely powerful body, escapes and wreaks havoc.