Joan Worthing has a new job as a seasonal ranger in Yellowstone National Park. She is hoping to escape a long history of physical abust 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 evasion. Doctor Rand Holland, a psychologist in new recovery from alcoholism, becomes involved in the search for Teddy and protection for 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 they hope will bring Satan to earth for the ultimate battle between good and evil.