Tearing its way through space on a collision course for Earth is Nemesis, a fiery ball of destruction, a dwarf star as red as the color of blood. Circling Nemesis is Rotor, an Earth colony whose occupants have cut themselves off from the anarchy and degeneration of an old and wasted world to form their own utopian existence. For them Rotor is a kind of Ark; one with hidden dangers that must be understood.

Set in the twenty-third century, this novel was written two years before Asimov's death, and is part of his unified History involving his Robot stories and the Empire series of stories. This story deals with a point in time just before the discovery of true FTL travel becomes possible.