Having left for Mars to search for an expedition dispatched some time ago, the astronauts take on board a single survivor, Colonel Edward Carruthers (Marshall Thompson), whom they suspect to have murdered the others in his crew. Carruthers insists that he is innocent and says that the real enemy and the author of the horrible massacre is a terrible alien creature. The rescuers become convinced that what the Colonel says is the truth, when brutal murders begin to take place inside the spaceship. "It" is on board and threatens the destruction of the small craft. The astronauts will have to think up something that will defeat the monster who thirsts for blood.

Not a terrible production, and considered by some to be the forerunner of "Alien" and "Queen of Blood", but low budget, to the point of re-using the spaceship from "Flight to Mars".
Under the monster costume designed by Paul Blaisdell is hidden the veteran Ray "Crash" Corrigan.