Poking around in a church cellar, a priest (Donald Pleasence) finds an otherworldly vial filled with slime. Frightened, he brings his discovery to a circle of top scholars and scientists, who eventually learn that the strange liquid is the essence of Satan. The slime then begins to seep out, turning some of the academics into zombified killers. As the possessed battle the survivors, student Kelly is infected by a large quantity of the liquid and becomes Satan personified.