Seth is a surveyor, along with his friend Theo, a leech-like creature running through his skull who tells Seth what lies to his left and right. Theo, in turn, relies on Seth for mobility, and for ordinary vision looking forwards and backwards. Like everyone else in their world, they are symbionts, depending on each other to survive.
In the universe containing Seth’s world, light cannot travel in all directions: there is a “dark cone” to the north and south. Seth can only face to the east (or the west, if he tips his head backwards). If he starts to turn to the north or south, his body stretches out across the landscape, and to rotate to face north-east is every bit as impossible as accelerating to the speed of light.
Every living thing in Seth’s world is in a state of perpetual migration as they follow the sun’s shifting orbit and the narrow habitable zone it creates. Cities are being constantly disassembled at one edge and rebuilt at the other, with surveyors mapping safe routes ahead.
But when Seth and Theo join an expedition to the edge of the habitable zone, they discover a terrifying threat: a fissure in the surface of the world, so deep and wide that no one can perceive its limits. As the habitable zone continues to move, the migration will soon be blocked by this unbridgeable void, and the expedition has only one option to save its city from annihilation: descend into the unknown.
Egan's latest work of experimental science fiction (after the Orthogonal trilogy) is impressively bizarre. He takes some of the physics concepts explored in Christopher Priest's The Inverted World and turns them up to 11, imagining a universe in which there are two dimensions each of time and space. Gravity works in wholly unfamiliar ways. Some people, called walkers, are born only able to look east and (if they bend backwards) west, but not north and south; they have a symbiotic relationship with siders, who live in walkers' brains and can look north and south and relay what they detect. Seth is a walker who shares his brain with Theo, a sider. The two of them work as surveyors, searching for the properly habitable zones into which their city, Baharabad, can be moved as its current zone becomes inhabitable due to the planet's rotation. (Baharabad is in constant motion, its forward edge being extended as its back edge is destroyed.) Egan provides copious and necessary papers on the math and physics of world (there's less information on the staggeringly weird biology), but even with that help, much of the science will make the plotting borderline impenetrable for anyone not already immersed in the concepts. Nonphysicists hoping to stay afloat by clinging to the plot will find there's little of it to hold onto. Egan may have out-Eganed himself with this one.