Joan Worthing has a new job as a seasonal ranger in Yellowstone National Park. She is hoping to escape a long history of physical and sexual abuse by her brother Teddy who is schizophrenic. Teddy escapes from a mental hospital and rushes to find her. He believes she must have his child who will save the world from an alien invasion. He is desperate to find Joan and save the world. Doctor Rand Holland, a psychologist in new recovery from alcoholism, becomes involved in the search for Teddy and the protection of Joan. One of Rand's teenage patients Lela Lander runs away from home seeking help from Rand. She becomes involved with a Satanic coven in Denver run by Lonn Majors.

All these characters meet in Yellowstone as the coven holds a Halloween ritual sacrifice that will bring Satan to earth for the ultimate battle between good and evil.