In remote Antarctica, a group of American research scientists are disturbed at their base camp by a helicopter shooting at a sled dog. When they take in the dog, it brutally attacks both human beings and canines in the camp and they discover that the beast can assume the shape of its victims. A resourceful helicopter pilot (Kurt Russell) and the camp doctor (Richard Dysart) lead the camp crew in a desperate, gory battle against the vicious creature before it picks them all off, one by one.

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 horror novelist Sutter Cane (Jürgen Prochnow) goes missing, insurance investigator John Trent (Sam Neill) scrutinizes the claim made by his publisher, Jackson Harglow (Charlton Heston), and endeavors to retrieve a yet-to-be-released manuscript and ascertain the writer's whereabouts. Accompanied by the novelist's editor, Linda Styles (Julie Carmen), and disturbed by nightmares from reading Cane's other novels, Trent makes an eerie nighttime trek to a supernatural town in New Hampshire.