Opening scenePanorama of the Mars in the distance we can see a wrecked spacecraft, close by another functional ship. A voice belonging to Colonel Edward Carruthers (Marshall Thompson) explains the first ship crashed 6 months ago, and hes the only survivor of the original crew. Because of the strange circumstances of the deaths, he fears his superiors on Earth may have doubts about his story.Back on Earth the press is briefed about the situation with the first Mars mission. The existence of a rescue second ship is revealed and that they have found Carruthers. It is explained that the survivor will be brought back for a court martial.During return preparations an airlock is found left open, we see the shadowy form of a strange creature enter the ship. The crew closes the airlock, unaware of the strange passenger. After successfully lifting off, the pilot Van Heusen (Kim Spalding) challenges Carruthers about the fate of the 9 dead crew members. Carruthers sticks to his story that the others were killed by something on the surface, and thats why they could not find any bodies Van Heusen shows Carruthers a skull recovered; a bullet hole can clearly be seen.The crew are openly hostile to Carruthers, with the exception of Ann Anderson (Shirley Patterson) who shows some kindness. She is not sure Van Heusen is right, despite the evidence of the skull. Carruthers feels he can trust Ann and begins to opens up about exactly what happened. Carruthers was driving the exploration jeep when they encounter a dust storm. Amid the confusion, the crew is picked off the jeep one by one. He and others could hear strange sounds and faint shadows moving through the dust. Carruthers thinks that someone shooting at the shape may have been responsible for the bullet hole in the skull.In another part of the ship Kienholz (Thom Carney) working alone hears a strange noise coming from a storage area. Curious, he investigates. Entering the room he hears a strange animal noise before being attacked and killed by the creature seen earlier. Higher up in the ship Carruthers thinks he hears something and goes to investigate with Eric Royce (Dabbs Greer) Kienholz cant be found and the crew begin an extensive search of the ship, Gino (Richard Hervey) gets left behind when he stops to light a cigarette, he is also attacked.The crew complete the search, as well as not finding Kienholz the realize Gino is also missing. Investigating one of the air vents Carruthers discovers Kienholz's body. Jack Purdue (Robert Bice) thinks Gino might also be in the same crawl space and goes to check. He finds Gino, near death. Before Purdue, can aid him, he in turn is attacked. Escaping, he orders the crew to seal the air vent.Heusen finds some grenades in storage and begin rigging the air vents. If the creature tries to break out it is thought the grenades should kill it. Once the vents are rigged, the crew withdraws to the rec area and begins assembling all the weapons they can find. Gino's brother Bob (Richard Benedict) expresses anger that no further attempt is to made to save Gino.During these discussions the crew begins to wonder if the creature may be intelligent. They are convinced its use of the air vents was not a random action.The creature triggers the explosives, and judging from the howling and enraged sounds the crew suspect it has been injured but not killed.Waiting for a while the crew investigate they find huge amounts of damage but understand they have now isolated the creature on that deck. Seeing an opportunity they corner the creature and open fire with guns. It is unaffected and the creature counter attacks driving the crew back to the rec room.Mary Royce remembers Gino making gas grenades in case dinosaurs where encountered on Mars. Mounting another attack the crew uses the grenades to retake the lower deck. Once more they are driven off by the creature. Mary Royce does an autopsy on Kienholz's body. She reports that every drop of moisture is gone. Even the bone marrow is powdery, and his glands are completely gone. This knowledge does not help Van Heusen who was injured in the last battle and has come down with a fever she cant controlStrange banging noises are heard coming from the deck controlled by the creature. Checking through a hatch door they discover the creature is exploring the ship, looking for weaknesses. As they consider their options, it is realized they can get around the creature by going outside the ship and re-entering a deck lower than the creature.Carruthers and Calder make the dangerous trip and re-enter the ship. Realizing they have the advantage they decide to rig a trap that will electrocute the creature. With the task completed they lure the beast into the trap, unfortunately the plan fails. Carrruthers escapes back through an airlock but Calder has broken his leg and is trapped in a corner with nothing more than a welders torch.The crew establishes contact with Calder, and he explains how he's been able to stop the creature. The gas bottle he is using only has a 3 hour supply. To add to their problems, Ann advises that without fresh blood supplies Van Huesen will dieFinelli and Purdue undertake the mission with Carruthers. Calder lets them know the creature is doing something in the reactor room down to the lower deck to get the blood. Calder lets them know the creature has entered the reactor room. They automatically close the only door and hope once again the creature is trapped.Huesen now delirious activates the controls that open the reactor shields in the hope of killing the creature. He considers the death of the four men a fair bargain to destroy the creature. The creature however senses something is wrong and breaks out of the reactor room before the shield is deployed. Finelli is caught off guard and killed. The other two men escape into the control room.The creature goes on a ramgage and begins randomly destroying things. Playing a hunch Carruthers tells Cadwell to crawl to the airlock. He then tells the others to get into spacesuits. The way to destroy the creature is to vent the atmosphere from the ship.The creature makes one last ditch effort to break into the area controlled by the crew. It finally succeeds just as Carruthers triggers the venting process. The crew brace themselves as the air threatens to drag them out in the venting process. The creature trapped by the same airflow fights to save itself till finally with the air gone, it dies