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.

In 1960, a pilot testing an experimental rocket powered aircraft accidentally flies into the future and finds himself in a sealed city whose people suspect he is a spy from outside their walls, but who want to keep him to procreate with the ruler's daughter because the majority of the inhabitants are sterile.