In 1973, the first manned expedition to Mars is marooned. By the time a rescue mission arrives, only the leader of the expedition is left alive. He is accused of murdering his crew and is taken into custody. As the rescue ship returns to earth, the real killer, a life-devouring monster that has stowed aboard, begins attacking the crew. The remaining crew must find some way to stop it before the ship returns to a panic-striken Earth.