Priest postulates an eerily enigmatic world where a city travels inexorably along tracks that are constantly picked up from behind and laid out ahead by members of an elaborate guild system. But why must the city keep moving toward the elusive optimum? Why must women from the surrounding countryside be lured to the city to give birth to one child and then be sent home again? And why must the city dwellers be shielded from the realities of their world, such as the peculiar shape of the sun? Through the eyes of a young initiate to the guild system, the answers gradually take form. The unwinding of this SF mystery is highly satisfactory, but the clever resolution is slightly deus ex futuristic machina.