Dr. Russell Marvin Hugh Marlowe and his wife Carol Joan Taylor are driving to work when a flying saucer fills the sky above. With only a tape recording of the ship’s sound to prove the encounter, Marvin—the man in charge of the American space program’s Project Skyhook, which has already launched ten research satellites—hesitates to alert his superiors. His father-in-law, General Hanley [Morris Ankrum], informs him that many satellites have since fallen back to Earth. Marvin confesses that he has lost contact with all of them and privately suspects alien involvement. The couple then witness the 11th satellite plunge from the heavens in the moments after its launch.
The next day, a saucer lands at the Skyhook facility in front of the main building. A group of aliens in metallic suits emerge, and one alien walks peacefully toward the entrance. He is met with gunfire from infantry guards, killing the solitary intruder, while the others and their saucer are protected by a force field. After retrieving the dead body, the saucer lifts off and returns fire, wreaking destruction across the facility and killing everyone except the Marvins, who remain trapped below ground. General Hanley is captured and carried away within the saucer. Too late, Marvin decodes a message on his tape recorder: the aliens simply wanted to meet with him, and their landing at Skyhook had been met with violence that could have been avoided.
Marvin reaches out to the aliens by radio and, along with Carol and Major Huglin [Donald Curtis], travels to meet them. They—accompanied by a pursuing motorcycle cop [Larry J. Blake]—are taken aboard a saucer, where the aliens extract knowledge directly from Hanley’s brain. The beings explain that they are the last of their species, having fled a destroyed solar system; they are extraordinarily aged and kept alive only by their protective garments. They have shot down all the launched satellites because they fear them as weapons. As proof of their power, they reveal the coordinates of a naval destroyer that opened fire on them and which they have since destroyed. Carol grows increasingly distressed, and the patrol officer, despite Marvin’s attempts to intervene, fires on the aliens; they subject him to the same mind-control process as Hanley. The aliens promise to return both Hanley and the officer. As the interaction continues, Carol spirals into irrationality while Marvin tries to stay calm. The aliens state that they will meet the world’s leaders in 56 days in Washington, D.C. to negotiate an occupation of Earth.
In his later observations, Marvin discovers that the aliens’ suits are made of solidified electricity and grant them enhanced auditory perception. He develops a counter-weapon against their flying saucers, which he successfully tests against a single saucer. As they escape, the aliens jettison Hanley and the patrol officer, who plummet to their deaths. Groups of alien saucers then attack Washington, Paris, London, and Moscow, and conventional weapons prove useless. But Marvin’s sonic weapon devastates the invaders, and the defenders learn that the aliens can be killed by small arms when they are outside the force fields surrounding their craft. After repelling the assault and delivering no further threats, Project Skyhook is reestablished with Dr. Marvin back in command.
