Derec and Ariel have escaped Dr. Avery and Robot City to find themselves on Earth. The reader gets to know what Earth is like, a few large underground cities with millions of people. So densely populated that there are still more people on Earth than on the eighty spacer worlds combined. We also find that on Earth there is a dislike of robots among most of the population.

Derec and Ariel are in Dr. Avery's apartment. Put there when they found a key of perihelion that took them there. They get help from R. David. The robot provides them with identification, ration cards, and information on how to blend in, or at least survive. They are trapped on Earth almost as much as they were trapped in Robot City. They have to search for a way off the planet, Ariel is getting sicker and needs to get some sort of medical attention, Wolruf and Mandelbrot are stuck in a spaceship around Kappa Whale and need to be rescued.

With fewer robots in this book the emphasis has shifted temporarily away from the three laws and more onto how the spacers can adjust to what Earth is throwing at them. There is still R. David, but his role is minor, being that he cannot safely leave the apartment.