Welcome to the strangest world in the history of science fiction! Seth is a topographer paired with Theo, a leech-like creature that lives in his head and helps him see what is to the left and to the right. Theo, in turn, relies on Seth for mobility, as well as front and rear vision. Like all the inhabitants of their world, they are a pair of symbionts who depend on each other for survival. In the universe where Set's world is located, light cannot spread in any direction: the directions to the north and east are occupied by the “dark cone”. Seth can only look to the east (and if he throws his head back, then also to the west), and when he tries to turn to the north or south, his body begins to stretch. Turning to face the northeast is a task as impossible as accelerating to the speed of light. The living beings that inhabit the world of Set are in a state of constant migration, which follows the shifting orbit of the Sun and the narrow habitable zone it has created. Cities are constantly being dismantled on one side and rebuilt on the other, while topographers draw up maps of future safe routes. Having joined the ranks of an expedition to the borders of the habitable zone, Seth and Theo are faced with a chilling threat - a rift on the surface of their world, so wide that no one I can’t even estimate its size. Meanwhile, the habitable zone continues to shift, and soon the migration will be stopped by an insurmountable gap. To save their city, the expedition members have only one thing left to do - plunge into the abyss of the unknown.