In 2029, aboard the United States Air Force space station ''Oberon'', Leo Davidson works closely with primates who are trained for space missions. His favorite simian co-worker is a chimpanzee named Pericles. With a deadly electromagnetic storm approaching the station, a small space pod piloted by Pericles is used to probe the storm. Pericles's pod heads into the storm and disappears. Against his commanding officer's orders, Leo takes a second pod and goes in pursuit of Pericles. Entering the storm, Leo loses contact with the ''Oberon'' and crashes on a planet called Ashlar in the year 5021. He discovers that the world is ruled by humanoid apes who speak English, use Domestication of the horse for transportation, and treat human beings as slaves.