A group of astronauts set out to stop a giant asteroid on a collision course with the planet Earth. They land on the asteroid, plant explosive charges and destroy it.
Afterwards they return to the staging area, a space station called Gamma 3 in orbit around the Earth. Unfortunately, a scientist from the mission has unwittingly carried a luminous-green substance on the leg of his spacesuit which quickly mutates into one-eyed, tentacled monsters with the ability to discharge lethal bolts of electricity.
The Gamma 3 crew fend off the alien creatures with their laser-based weaponry, only to discover the creatures feed off the energy which, in turn, allows them to multiply rapidly, sprouting the new creatures from their blood. As the creatures overrun the station the crew continues to fight back against overwhelming odds.