Having just returned from a mission to Mars, Commander Ross (Justin Lazard) isn't exactly himself. He's slowly becoming a terrifying alien entity with a one-track mind - to procreate with human women! When countless women suffer gruesome deaths while bearing half-alien offspring, scientist Laura Baker (Marg Helgenberger) and hired assassin Press Lennox (Michael Madsen) use Eve (Natasha Henstridge), a more tempered alien clone, to find Ross and his virulent brood. But they underestimate Eve's maternal drive, and before long she escapes to mate with Ross in order to create a pure, unstoppable race that could spell doom for mankind. As time is running out, the ultimate battle takes place in a chilling climax that puts Darwin's theory to the ultimate test... only the strongest species will survive.