Synopsis:Rone is a goat herder in a village constructed on ledges within a vast pit that seems to have no bottom or top. While he is still a boy, the Sun Thread, from which all light and heat in the Pit come, goes out for a short time, and gravity itself stops with it. Rone, reckless with protecting his family's livelihood and dismissive of his best friend Heng's fears, loses both goats and Heng to the pit when the Sun Thread comes back to life and gravity returns.
Grown to a young man, Rone is haunted by the loss of his childhood friend but is distracted from those thoughts by Heng's beautiful cousin Eni. Eni's family, apart from Eni herself, despises Rone because of his part in Heng's death.
Eni's uncle Huhnun falsely accuses Rone of a crime, and Rone is exiled. Climbing down the rock walls of the pit, he travels to another village where he works to build support for clearing his name back at home. Before he can return, Huhnun appears and tries to kill Rone to prevent being exposed as a liar. They fight and fall together into the Pit. Huhnun hits the rock wall and is killed.
Much farther down, Rone is caught by a huge net stretching across part of the Pit opening at a great cliff city, where miners get metal from deep in the rock and traders use gliders to travel to settlements further down-Pit.
Rone is held as a captive by the city-dwellers, but he escapes with a glider and spirals deep into the Pit, seeking its end. Exhausted, he finally stops to rest and discovers a room full of advanced technology, including an artificial intelligence. The AI reveals the origin of the Pit and the Sun Thread, which is breaking down. If Rone, the first human in ages to reach that place, doesn't diagnose and repair the Sun Thread, soon it will go out entirely, extinguishing all light and gravity. The task seems hopelessly complex, and although the computer can teach Rone, Rone doubts he will ever understand enough to be of any use.
But the computer also reveals that the Pit is actually a huge torus, with the Sun Thread pulling objects down along its length. Originally the Sun Thread pushed out instead, making the entire inner surface of the torus habitable, but it malfunctioned and turned that entire miniature world into a bottomless pit. Rone realizes that if he continues down, he'll eventually reach the place he started: he can go home. Inspired by this unexpected chance, he throws himself into learning the material he needs to master in order to fix the Sun Thread.
As he comes to understand the workings of the torus, Rone realizes that it's the miners at the city where he was held captive that are causing the Sun Thread to fail by pulling metal from machinery that had been hidden under the rock. He travels back to his home village, where over the protests of skeptics he is recognized as a Kumam, a spiritual leader--a person who can fly. He is reunited with Eni and gathers a group to build gliders and fly to the city, where they use the controls Rone can access through the AI to force the city-dwellers to realize what has happened, stop the mining, restore the machinery, and save the Sun Thread.