1950s sci-fi horror. Aboard a nuclear-powered spaceship, on the return journey from Mars, Col. Edward Carruthers (Marshall Thompson) is left as the sole survivor after his fellow crew members are all killed by a hostile alien life form. A rescue mission, headed by Commander Col. Van Heusen (Kim Spalding), is sent in to investigate the disaster but they too begin to be picked off by the seemingly invincible monster.