Four astronauts returning from man's first mission to Mars enter a time warp and crash on a 26th-century Earth devastated by atomic war. At first unaware where they are, but finding the atmosphere safe to breathe, they start exploring and find themselves in a divided future where disfigured mutants living like cavemen inhabit the surface, while the normals live comfortably below the surface but are dying as a race from lack of natural water, air and sunlight.