The Earth is attacked by mysterious invaders from outer space, who plan on destroying humankind. The invaders are invisible in our atmosphere, but are able to inhabit and reanimate the bodies of the dead. The armies of rotting corpses march on the cities, and it seems as though there is no defense. Major Bruce Jay (John Agar) is put in charge of a small, secret research center with a group of scientists, who must find a way of combating the invaders. Personality conflicts develop as Jay's hard-nosed, by-the-book approach to his job -- which requires him to kill anyone who might jeopardize the mission -- put him in opposition to the scientists (played by Jean Byron, Philip Tonge, and Robert Hutton). They develop an ultra-sonic gun that has the combined effect of rendering the aliens visible and killing them, but first they must test it, by capturing an alien, an action that forces them to run the risk of being discovered.