This novel is perhaps the hardest science fiction ever published by Egan, filled with non-trivial mathematics and theoretical physics. Twenty-thousand years in the future, Cass, a humanoid physicist from Earth, travels to Mimosa orbital station and begins a series of experiments to test the extremities of the fictitious Sarumpaet rules, a set of fundamental equations in "Quantum Graph Theory," which holds that physical existence is a manifestation of complex constructions of mathematical graphs. However, the experiments unexpectedly create a bubble of something more stable than ordinary vacuum, dubbed novo-vacuum, that expands outward at half the speed of light as ordinary vacuum collapses to this new state at the border, hinting at more general laws beyond the Sarumpaet rules. The local population is forced to flee to ever more distant star systems to escape the steadily approaching border, but since the expansion never slows, it is just a matter of time before the novo-vacuum encompasses any given object.
Two factions develop as the expanding bubble swallows star after star: the Preservationists, who wish to stop the expansion and preserve the Milky Way at any cost; and the Yielders, who consider the novo-vacuum to be too important a discovery to destroy.
Six hundred years after the initial experiment, aboard the Rindler, a vessel that has matched velocities with the border, a variety of refugees are probing the novo-vacuum in order to understand the physics that makes it possible. The novo-vacuum turns out to be more complicated than anyone suspects, however, and Egan's usual topics of simulation and quantum ontology are taken to the extreme when we learn that a whole ordered universe exists within this zone of apparent chaos, existing as direct elaborations of the quantum graph's lattice structure, of which elementary particles, fundamental interactions, and spacetime itself are only special cases.