For twenty thousand years, every observable phenomenon in the universe has been successfully explained by the Sarumpaet Rules: the laws governing the dynamics of the quantum graphs that underlie all the constituents of matter and the geometric structure of spacetime. Now Cass has stumbled on a set of quantum graphs that might comprise the fundamental particles of an entirely different kind of physics, and she has travelled three hundred and seventy light years to Mimosa Station, a remote experimental facility, in the hope of bringing this tantalising alternative to life. The “novo-vacuum” is predicted to begin decaying the instant it’s created, but even a short-lived, microscopic speck could shed light on the origins of the universe, and test the Sarumpaet Rules more rigorously than ever before.

Cass’s experiment turns out to be more successful than anticipated: the novo-vacuum is more stable than the ordinary vacuum around it, and a region in which the new physics holds sway proceeds to expand out from Mimosa at half the speed of light.

Six hundred years later, more than two thousand inhabited systems have been lost to the novo-vacuum. On the Rindler, a ship that has matched velocities with the encroaching border, people have come from throughout inhabited space to study the phenomenon. Most are Preservationists, hunting for a way to turn back the tide, but a few belong to another faction: Yielders, who believe that the challenge of adapting to survive on the far side of the border would reinvigorate a civilisation that has grown stale and insular.

Tchicaya has come to the Rindler to join the Yielders, but when Mariama — a childhood friend whose example inspired him to abandon his own home world and traditions for a life of travel — arrives soon after, he is shocked to discover that she plans to help the Preservationists find a way to destroy the novo-vacuum.

As a theoretical breakthrough leads to a sequence of experiments that begins to reveal the true richness of the world behind the border, tensions between the opposing factions grow. When a splinter group responds to these revelations with violent, unilateral action, Tchicaya and Mariama are forced into an uneasy alliance, and travel together through the border, balancing old and new loyalties against the fate of two incomparably different universes.