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In 1973, the United States Space Commission announces that a spaceship under the command of Van Heusen has been sent to rescue the first spacecraft to land on Mars, after receiving notification from its commander, Col. Edward Carruthers, that he is the lone survivor of his crew of nine. Under suspicion for murdering the crew, Ed is being brought back to Earth for a court-martial. On Mars, as the rescue ship prepares to blast off, an alien boards the ship through an open hatch and hides in the storage compartment. Van and most of the crew are wary of Ed's presence, doubting his story that while returning to the ship from a day of exploration, his crew was chased by an unidentified creature and subsequently disappeared in a sand storm. While working in the lower part of the ship, crew member Joe Keinholtz hears a strange noise and, upon investigating, is attacked and killed by the Martian. Later, after crew member Gino Finelli does not return from a trip to the lower deck, his brother Bob grows concerned and demands that Van and the others look for him. Below deck, the pale, limp body of Keinholtz is discovered, and Jack Purdue finds Gino barely alive, wedged in an air duct, but is threatened by the Martian before he can reach him. After Purdue escapes, Ed orders the hatch sealed and mined with grenades. The crew listens as the hatch rattles and the grenades explode, but afterward they are amazed when the sounds of the alien crashing about are heard over the radio speaker. After Van orders the men below to investigate, they find the lower deck smashed. When the alien attempts to jump out of a hatch, the men shoot him several times to no avail. Back on the upper deck, nurse Ann Anderson suggests they attack the creature with gas. The lower deck is flooded with fumes, but when Van opens the hatch to check the results, he is clawed by the alien. Nurse Mary Royce cannot immediately determine Keinholtz's cause of death, but an autopsy reveals that Keinholtz did not die from being crushed, but from all the fluid in his body being removed. After Mary's husband Eric suggests they electrocute the alien, Ed and crew member Jim Calder reach the deck below the alien by climbing outside and down the side of the ship. While the remaining crew make noises to distract the Martian, Ed and Calder rig the trap on the stairwell. The crew is dismayed when the Martian is caught as planned but barely scathed by the jolt. Angered by the attack, the creature attacks Calder, who is injured and urges Ed to escape without him. Calder holds off the creature's subsequent attacks with a blowtorch as Ed rejoins the others and plans another strike. Overwrought, Van accuses Ed of having abandoned Calder. Hearing the accusation over the ship's speaker, Calder refutes Van. Mary then reports to Ed that Purdue and Van have developed leukemia-like symptoms from their attacks and she has had to transfuse them with all the blood available on the main deck. As Ed decides that he and Eric must risk going below for more blood, Calder reports that the alien has taken Gino's body to the reactor room. Van orders the core be exposed in an attempt to irradiate the creature as Ed, Eric and Bob go to the lower storage area to get fresh blood. The radiation attack infuriates the Martian, who then attacks and kills Bob. The crew members flee to the top deck, certain that it is only a matter of time before they are all murdered by the alien. After Ed notices the high oxygen levels in the ship, Eric speculates that the thin air of Mars has provided the creature with strong lungs. Ed then orders the crew to don oxygen masks and drains the entire ship of oxygen, which at last kills the Martian. Back on earth, the Space Commission receives a request from Van to drop the charges against Ed and a warning from the remaining crew that Mars should remain off limits for all future expeditions.