On the planet Kalgash scientists from the fields of psychology, archaeology, and astronomy have recently made discoveries that are profoundly disturbing in their implications. The people of Kalgash live their entire lives in sunlight; their planet orbits multiple suns, at least one of which is always in the sky. They have little experience with darkness. When an amusement park ride offers the thrill of several minutes of darkness, some riders suffer nervous breakdowns. A psychologist who specializes in the effects of darkness on the mind is called in to investigate the ride. His discoveries have dire implications when combined with what archaeologists and astronomers have found.

A team of archaeologists who have been studying the oldest known city on their planet by chance discover an even older city. Under that city is another one, and then another below that one, and so on for several layers...