Out beyond the orbit of the moon there was a huge, wheel-shaped space station. Its rapid spin pressed the equivalent of one Earth gravity against its broad, thick rim. Once when the distortion field failed, the Mt. Palomar telescope tracked it for the better part of an hour, but earth astronomers attributed the track either to an irregularity in the photographic plate or to some peculiarity in the atmosphere.

Near the hub where the gravity was weak, the nine aliens lived; in the two rim compartments lived the mutants. There were almost a thousand of the latter—both male and female—in the larger compartment; and fewer than thirty—all male—in the smaller one.

"Soon, now," the mutants told each other with growing excitement, "we shall go down and kill them."

The aliens stepped up the power in the larger of the two transmitters. "Our indoctrination is perfect," they reassured themselves. "The mutants will not get out of hand."