Dr. Roger Girard, an ambitious and morally ambiguous scientist, lives in a remote estate where he conducts bizarre medical experiments. Obsessed with transplanting heads and bodies to create hybrid beings, he constructs a crude laboratory in the basement with the help of his assistant. One evening, Manuel Cass, an escaped mental patient and dangerous murderer, breaks into the estate, seeking refuge. Simultaneously, Danny, a mentally-disabled farmhand resembling Cass, arrives to seek work. Girard takes a twisted opportunity when both men are incapacitated; he decides to transplant Cass’s head onto Danny’s large and powerful body, hoping to attain a creature with physical strength and intelligence.

The experiment succeeds but has horrifying consequences as the two-headed being possesses strength and violent tendencies influenced by Cass’s criminal mind. The now-demented creature breaks out of control, wreaking havoc and leaving a trail of destruction that Dr. Girard struggles to contain. The local authorities and townspeople become involved as the chaos escalates, making it increasingly difficult for Girard to hide his ghastly creation. Tension mounts as the creature's rampage threatens everyone in its path.