Plot summary

George Gardner (Timothy Bottoms) encourages his 12-year-old son David's (Hunter Carson) dreams of becoming an astronaut by stargazing with him. A thunderstorm wakes David, and he observes a strange alien spaceship landing on Copper Hill, just beyond the house. His father agrees to investigate, but returns a changed man.In the morning, David Gardner notices his father wearing only one slipper. His normally sunny disposition is somber and his personality is flat. David also notices a wound on the back of his father's neck in the shape of a letter 'X'. In the kitchen, George Gardner dumps sugar cubes into his coffee, and the coffee dribbles down his chin as he drinks. David is frightened by his father's change in behavior. At night, when he confesses his suspicions to his mother, Helen Gardner (Laraine Newman), she assures him that George is fine. Later, however, Helen calls police after she realizes George is missing.When the police chief (Jimmy Hunt) and Officer Kenney (Kenneth Kimmins) agree to investigate Copper Hill, George suddenly appears. The officers return to the house with altered personalities, but only David notices the difference. He wakes up in the middle of the night, and observes as his parents disappear over the ridge. In the morning, Helen is transformed. She offers David a plate piled with burnt bacon, and wants to take him to see the special place on Copper Hill. When David sees his mother eat a chunk of raw hamburger smothered in salt, he declines to go.At school, David overhears his schoolteacher, Mrs. McKeltch (Louise Fletcher), tell the police chief about a plan that involves George Gardner. David sneaks into Mrs. McKeltch's office and catches her eating a live frog. Mrs. McKeltch chases David, claiming he knocked over a female student. Nurse Linda Magnusson (Karen Black) rescues David and questions him in her office. She has no neck wound, so David gives his account of the unidentified flying object (UFO) at Copper Hill that robs people of their personalities. When Linda confronts Mrs. McKeltch, the teacher refuses to let the nurse examine the "boil" on her neck. Linda also notices a bandage on the neck of a student named Heather (Virginia Keehne).Linda fears for David's safety and suggests he hide at her house. However, David evades Mrs. McKeltch and Heather as they search for him in the parking lot, and becomes trapped when he hides in the teacher's van. She drives to a cave, and David follows her inside. After walking through a series of tunnels, Mrs. McKeltch encounters a round alien creature, a face and brain uttering sounds. As it speaks, light flashes from the device planted in Mrs. McKeltch's neck. The aliens realize David is in their midst, and Mrs. McKeltch vows to "get him" before he escapes.Soon, Linda Magnusson finds David on the run, and he shows her the alien cave. When he gets to the location, however, the cave has disappeared. He insists they go to Copper Hill, but the spaceship is also gone. In the distance, they hear George Gardner send two men with metal detectors into the sandpit at Copper Hill. David and Linda watch as a circular opening in the earth swallows the men. Linda and David drive away and narrowly miss hitting a school bus driven by Mrs. McKeltch.When Linda stops at a gas station to call state police, Mrs. McKeltch kidnaps David and drags him toward the school bus, but he breaks free. Linda hears the disturbance, and David gets into her car. They arrive at her office to find the telephone inoperable. The chief and Officer Kenney search the building while David leads Linda to the basement furnace room. They knock over some equipment, alerting their pursuers. However, an alien spaceship emerges through the floor, causing the officers to lose their footing, and enabling David and Linda to escape.David wants to find his parents before leaving town, but Linda insists they get help. David and Linda meet with General Wilson (James Karen), commander of the military base that employs David's father. The general attempts to verify their story by questioning the men with metal detectors, but the interrogation is halted when an invisible force kills the prisoners. The neck devices emit sparks of light as they appear to unscrew from the necks of the corpses. The general orders the base perimeter sealed, and summons officials from the National Aeronautics and Space Administration (NASA).NASA and Search for Extraterrestrial Intelligence (SETI) scientists meet with the general and insist that their launch to Mars go as scheduled. For security purposes, the general calls for a temporary delay until they can determine the risks. Meanwhile, George Gardner plants a bomb inside a truck bound for the launch pad. A monitor in the general's office indicates that the bomb has successfully destroyed the Mars rocket.It is here that General Wilson finally believes David's claim about aliens and speculates that the aliens are from the nearby planet Mars and when they observed the SETI rocket under construction and bound for Mars, the aliens misinterpreted it as an attack on their planet and so they decided to strike first by landing on Earth to destroy the rocket before it could be launched. General Wilson asks David to show him the sandpit at Copper Hill, and orders his soldiers to surround the area, but Sargent Rinaldi (Eric Pierpoint), the general's aide, loses his footing and the sandpit swallows him.Two aliens stand near the cave, eyeing advancing soldiers and Dr. Mark Weinstein (Bud Court), a member of SETI. Weinstein wants to converse with the aliens before the military launches an attack. He returns an alien mechanical part, believing the aliens understand him. Nevertheless, the scientist is vaporized by the alien's ray gun, and the soldiers kill the aliens.David runs into the sandpit hoping to locate his parents, and Linda follows. Both are swallowed up, and the general sends soldiers after them. They lay down explosives and rappel into the hole left by the explosion.Inside a lab at the alien spaceship, David observes as a device is implanted in Sgt. Rinaldi's neck to control humans. Mrs. McKeltch orders other aliens to capture David and Linda. David pleads with the Martian Supreme Being to release his parents, arguing that mind control is wrong, and predicting the aliens' failure in conquering Earth. The head Martian comments that David is "a poor little guy," and the boy punches him in the nose. As he runs away, David knocks over Mrs. McKeltch, who falls into the gaping mouth of an alien and is eaten alive.In the tunnel, realizing Sgt. Rinaldi is under the control of the Martians, the general orders his men to shoot him dead.David leads the soldiers back to the spaceship to destroy the Martian Supreme Being and save Linda from receiving an implant. Many soldiers are vaporized, but American firepower causes the Martian Supreme Being to retreat into a protective eye-shaped shell. As Linda is saved, soldiers plant explosives, and the general orders a five-minute delay to escape before detonation.Upon discovering that the escape route is blocked, David sacrifices a mint copper penny his father gave him to activate an alien weapon that is left behind since the metal copper is the source of the alien's technology. Meanwhile, the wounded Martian Supreme Being orders the aliens to prepare the ship for takeoff.The soldiers escape through the new opening, but David hears his parents calling. He refuses their entreaties to join them on the spaceship, and while he struggles against them near the ridge, the spaceship explodes in the sky. The implants in his parent's necks are apparently deactivated, but the aerial fireball begins to overtake and consume both David and his parents....Suddenly, David awakens in his bedroom. His parents comfort him, and he is relieved to find nothing implanted in his father's neck. The entire Martian invasion and battle was apparently nothing more than David's nightmare. His mother tucks him into bed and reminds him that dreams are not real and tells him goodnight. But sometime later, a thunderstorm snaps him awake. When he opens his window, David sees the exact same Martian UFO spaceship landing on Copper Hill. He runs to his parents' room in a panic. David's nightmare is starting again.... or is it?