Scientist Clayton Forrester (Gene Barry) and Sylvia Van Buren (Ann Robinson) are the first to arrive at the site of a meteorite crash. Soon after, an alien war machine emerges and begins killing at random. The Marines are called in, but they're no match for the aliens' force field. Forrester and Van Buren, however, are able to wound one of the creatures and procure a sample of its blood. They take it to Los Angeles where they hope, through testing, to be able to discover the aliens' weakness.