Too terrifying to even have a name, "It" is a seemingly invincible monster that is hell-bent on killing everyone on a mission to Mars. A rescue ship travels out to Mars to retrieve the only survivor of a space probe that has experienced some sort of cataclysm. That survivor, Col Ed Carruthers (Marshall Thompson), is accused of murdering his fellow crewmen. But Ed claims that the killer was a Martian monster, and hopes to prove his assertions by signing up for a second journey to the Red Planet. Before long, the crew members of this second expedition are being systematically killed off, and it looks as though Ed is up to his old tricks. As it turns out, however, Ed was telling the truth: there is a monster on board, the savage descendant of the once-mighty Martian civilization, who snuck on board when an irresponsible crew member left the door open. The monster stays alive by absorbing the vital body fluids of its victims - and there seems to be no way to stop this parasitic creature! Will they be able to destroy the monster before it manages to feed on them all?