Une comète doit très prochainement passer près de la Terre. Les calculs savants des scientifiques indiquent que l'astéroïde ne devrait pas entrer en collision avec la planète bleue. Elle ne fera que la frôler, causant quelques interférences magnétiques. Mais un astrophysicien, le Dr James Mayfield, chargé d'étudier ces perturbations, en vient à penser que la comète pourrait tout de même s'écraser sur la Terre. C'est alors qu'un fragment se détache et tombe sur l'Alaska...
A group of astronauts set out to stop the asteroid Flora, which is 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.
While reviewing pictures of the planet Jupiter, astronomer Jennifer Dreiser (Joanna Miles) discovers a new comet streaking through space. The joy of finding the comet is short-lived, however, when it is determined that it may hit the Earth in just eight days. Dreiser is summoned to see the President (Andrew Duggan) at his vacation home, along with a number of other experts, to discuss the situation. Astronomer Jason Voight (Richard Crenna), a specialist in near-Earth objects, shocks the group by telling them that the comet will hit within 50 miles of Phoenix. Voight urges the President to order an evacuation of Phoenix and the surrounding area, but the President ultimately leaves that decision in the hands of Arizona's governor, Michael Ritchie (Nicolas Coster). The President does order the MIRV nosecone of a Minuteman ICBM be launched into space aboard a Titan-Centaur rocket, so that an attempt can be made at destroying the comet.