One hundred thousand years ago. In the wake of the apparent self-destruction of the alien Forerunner empire, two humans-Chakas and Riser-are like flotsam washed up on very strange shores indeed. Captured by the Forerunner known as the Master Builder and then misplaced during a furious battle in space, Chakas and Riser now find themselves on an inverted world, where horizons rise into the sky and humans of all kinds are trapped in a perilous cycle of horror and neglect. They have become both research animals and strategic pawns in a cosmic game whose madness knows no end-a game of ancient vengeance between the powers who seeded the galaxy with life, and the Forerunners who expect to inherit their sacred Mantle of Responsibility to all living things. In the company of a young girl and an old man, Chakas begins an epic journey across a lost and damaged Halo ringworld in search of a way home, an explanation for the warrior spirits rising up within, and the reason for the Forerunner Librarian's tampering with human destiny. Their travels will take them into the domain of a powerful and monstrous intelligence-known as "the Captive" by Forerunners, and "the Primordial" by ancient human warriors, this being may not only control the fate of Chakas, Riser, and the rest of humanity, but of all sentient life across the galaxy...