Above a certain planet, which its inhabitants simply call the World, orbits another one—the Upper World—touching the World with the upper layers of its atmosphere. According to the prevailing religious doctrine, there exists the same civilization there as on the World, and the souls of the dead are embodied in those who live there, and vice versa. However, the World faces a great danger: pterta, an unknown form of life in the shape of flying spheres, infects people with a fatal disease that rapidly reduces the population. To save at least someone, scientists create ships capable of reaching the Upper World, but not everyone agrees to move there.