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 (Susan Blanchard) is infected by a large quantity of the liquid and becomes Satan personified.

When his longtime partner on the force is killed, reckless U.S. Secret Service agent Richard Chance (William L. Petersen) vows revenge, setting out to nab dangerous counterfeit artist Eric Masters (Willem Dafoe). Along with his new, straitlaced partner, John Vukovich (John Pankow), Chance sets up a scheme to entrap Masters, resulting in the accidental death of an undercover officer. As Chance's desire for justice becomes an obsession, Vukovich questions the lawless methods he employs.