Young David Gardner (Hunter Carson) lives with his parents in a rural area near the Marine Corps facility where his father works as a NASA research scientist. During a violent thunderstorm, David is startled awake by a blinding flash of light. From out his bedroom window he observes a flying saucer landing behind the nearby hill. Frightened, David runs to tell his mother, Ellen (Laraine Newman), and father, George (Timothy Bottoms) and his father reluctantly agrees to investigate. Arriving back at the house, George reports that nothing is there. But his manner seems changed - cold and remote. The boy also notices a small bloody mark, much like an "X", at the back of his father's neck. George later takes his wife up the hillside and she too returns changed. With the help of the school nurse (Karen Black) and the U.S Marines they must fight to save Davis parents and the village from being controlled by Martians.