In the wake of the apparent self-destruction of the Forerunner empire, two humans - Chakas and Riser - are like flotsam washed up on very strange shores indeed.

They find themselves on an inverted world where horizons rise into the sky and where humans of all kinds are trapped in a perilous cycle of neglect. They have become 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.

In the company of a young girl and an old man, Chakas begins an epic journey across a lost and damaged Halo in search of a way home, an explanation for the warrior spirits rising up within, and for the Librarian's tampering with human destiny.

This journey will take them into the domain of a powerful and monstrous intelligence who claims to be the Last Precursor, and who now has control of both this Halo and the fate of Forerunners and humans alike.

Called the Primordial by ancient human warriors, this intelligence may control the fate of not only Chakas, Riser, and the rest of humanity, but all of sentient life.