In the novel, Asimov depicts the transition from his earlier Milky Way Galaxy, inhabited by both human beings and positronic robots, to his Galactic Empire. The galaxy of his earlier trilogy of "Robot" novels is dominated by the blended human/robotic societies of the fifty "Spacer" planets, dispersed over a wide part of the Galaxy. While the Earth is much more populous than all of the Spacer planets combined, its people are looked down upon and treated almost as sub-human by the Spacers. For a long time, the Spacers have forbidden immigration of people from the Earth. But Asimov's later Galactic Empire is populated by many quadrillions of human beings on hundreds of thousands of habitable planets, and by very few robots (such as R. Daneel Olivaw). Even the technology to maintain and upgrade robots exists on only a few out-of-the-way planets. Therefore, Asimov's novel attempts to describe how his earlier "Robot" series ultimately connects to his "Galactic Empire" series.