The setting and notion behind this novel first appeared in a short story of the same title published in New Writings in SF 22 (1973), though the idea was more fully developed in the novel. The setting is Earth City, a curious structure that is winched along a set of rails that are perennially being removed from behind the city and replaced in front. The city must keep moving or it will suffer a terrible fate. What this fate is, and why, is revealed only gradually during the course of the novel.
Helward Mann, having reached adulthood, is apprenticed to the Future guild, though he must first spend time working with each of the other guilds that are vital to the citys survival. He begins with the track layers and learns that the city must keep moving in pursuit of “the Optimum,” though he does not yet know what the Optimum might be. He also discovers that the sun is a flattened disc from which infinitely long spikes protrude top and bottom.
Later, Helward learns more about the threatening nature of the world when he escorts a group of native women back to their village. The further south from the city he goes, the more the landscape is distorted. A deep chasm that delayed the city while a bridge was built has turned into a narrow creek. The women start to grow shorter and fatter. He himself feels an almost irresistible pull, like centrifugal force, that nearly sweeps him away. There are time distortions as well, so that a journey of only a few subjective days in fact takes many months.
Helward pieces together the information he has about the city and concludes that its inhabitants are survivors of an expedition from Earth on a hyperboloid planet where the ever-moving optimum is the only place where Earthlike conditions obtain. Elizabeth, whom Helward meets on one of his expeditions north of the city, reveals a different, more devastating truth: They are actually on Earth and are survivors of an experiment with a different sort of power made at the time of “the Crash.” The experiment has inverted their perceptions of their world.