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n the last day of their mission to Mars, three astronauts - Ed Barkley, Alan Wells and Pete Claridge - discover a cave with hieroglyphics on one wall. They also find a pod-like object but they are knocked out after trying to move it. They make it back to their ship and begin their voyage home. Five months into that return trip they begin to have equipment malfunctions and find a small growth of alien origin. Soon, one of the astronauts begins to believe that the others are aliens who have taken over the bodies of his shipmates.

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