In Medieval France a warlock is beheaded and his wife is tortured and executed. Hundreds of years later, an isolated group of people discover his head buried on their property. Soon it comes back to life, possessing people and using them to commit sacrifices and to search for the rest of his body.

A scientist comes to believe that evil is a disease of the blood and that the flesh of a skeleton he has brought back from New Guinea contains it in a pure form. Convinced that his wife, a Folies Bergere dancer who went insane, manifested this evil he is terrified that it will be passed on to their daughter. He tries to use the skeleton's blood to immunise her against this eventuality, but his attempt has anything but the desired result.

The Earth is invaded by alien parasites—AKA 'slugs'—that ride on people's backs and control their minds.