In 1973, a nuclear power spaceship blasts off from Mars for Earth, bringing with it the sole survivor of the first mission, Col. Edward Carruthers (Marshall Thompson). He is suspected of having murdered the other nine members of his crew for their food and water rations, on the premise that he had no way of knowing if or when an Earth rescue mission would ever arrive. Carruthers denies this allegation, attributing his crew's deaths to a hostile humanoid life form on the Mars.