David MacLean [Jimmy Hunt] is awakened by a violent thunderstorm one night and witnesses a flying saucer descend into the sandpit behind his home. He rushes to tell his father, George MacLean [William Phipps], who goes to investigate. By morning, the father returns altered—an odd puncture marks the back of his neck, and he behaves coldly and distant, insisting that David tell no one about what he saw. When Mary MacLean [Hillary Brooke], the boy’s mother, searches the area, she too encounters the same unsettling change in the people around her. The morning after the sighting, the town’s routine seems unnerved, with authorities and neighbors acting as if nothing happened, and the warning signs multiply as the adults around David begin to slip under an invisible influence. A neighbor child, Kathy Wilson [Janine Perreau], vanishes underground in the sandpit only to return later and set her own house ablaze, leaving a chilling impression of something lurking just beneath ordinary life.
Desperate for help, David goes to the police, only to find the head of the station as affected as the others; the chief refuses to acknowledge anything out of the ordinary. A desk officer brings in Dr. Pat Blake [Helena Carter], a physician who, after reviewing David’s account and noting the lack of typical explanations, starts to trust his story. With her guidance, they seek out Dr. Stuart Kelston [Arthur Franz], an astronomer who helps them interpret the strange—yet increasingly undeniable—clues. Kelston suggests that a Martian invasion could be imminent, aided by the proximity of Mars in Earth's sky, and he proposes a troubling hypothesis: the nearby government research plant developing an atomic rocket may threaten Martian life if their experiments go awry. To test this theory, Kelston reaches out to the U.S. Army, and soon a military response is organized under Colonel Fielding [Morris Ankrum]. The Army traces a string of signals back to the sandpit and closes in on the landing site, surrounding the saucer with tanks and troops while tension thickens the air.
As the martial hunt unfolds, Dr. Blake and David are suddenly pulled underground by tall, green, slit-eyed beings and taken through a network of tunnels directly to the saucer. Army forces locate the tunnel entrance and charge inside. Colonel Fielding leads a small contingent toward the alien craft, where they confront the Martian supreme intelligence. Under the Martians’ mental commands, a cadre of silent mutants patrol the interior and maintain control over their victims with mind-control devices embedded at the base of the skull; should anyone be captured, the devices unleash a fatal cerebral hemorrhage. The encounter becomes a tense battle as the human group fights their way through the tunnels, shooting and scrambling toward an exit.
Fielding’s unit plants timed charges on the saucer, and a desperate rush to evacuate ensues. They force a path to the surface, using a Martian excavation tool to blast a tunnel back to daylight. David, fleeing downhill, is overwhelmed by a flood of alien images as his mind absorbs a broader picture of Martian strategy and invasion—visions that include events he did not personally witness, yet now feel inexorably connected to the night’s events.
When the saucer finally explodes, David awakes back in his own bed on the stormy night. He hurries to his parents’ room, terrified and confused, and is reassured that it was all just a nightmare. Yet as he returns to his own bed, he again glimpses the same saucer descending into the sandpit, leaving the unresolved fear of a first contact that has not yet ended. The line between dream and reality blurs as the memory of the invasion lingers in David’s mind.
