Panic breaks out at the United Nations Space Authority when it is discovered that an asteroid has steered out of orbit and is heading for a fatal collision with Earth. Since the only way the catastrophe can be averted is to blow up the asteroid from the space station Gamma III, astronaut Jack Rankin is sent there to take charge of the mission. Although the station's commander, Vince Elliot, is a former friend of Rankin's, friction has developed between the two men because Rankin does not consider Elliot fully capable of leadership, a situation that is not helped by the fact that Elliot is now engaged to Rankin's former girl friend Lisa Benson, one of the station's doctors. Despite their hostility, the two men embark on a spaceship flight and land on the asteroid. While explosives are being set up, a scientist named Doctor Halvorsen explores a swamp filled with a jelly-like green substance that seems to be alive. After the asteroid has been destroyed and the spaceship has returned to Gamma III, Doctor Halvorsen discovers that some of the green slime that adhered to his sleeve has become activated into a blood-like substance that rapidly transforms itself into a tentacled monster capable of emitting lethal electrical shocks. When the creature kills Halvorsen, Rankin shoots it only to find that the drops of blood it emits quickly evolve into new monsters. Realizing that the more creatures they kill, the more they will create from the drops of blood, Rankin and Elliot are forced to accept the fact that Gamma III must be evacuated and blown up before the green plague spreads to Earth. Although they succeed in destroying the space station and the monsters, Elliot pays for it with his life.