Facing the imminent collapse of their civilization, the Forerunners known as the Ur-Didact and the Librarian reveal what they know about the relationship between the long-vanished race of the Precursors and the Flood. While the Precursors created many technological species, including those of the Forerunners and humanity itself, the roots of the Flood may be found in an act of enormous barbarity, carried out beyond our galaxy ten million years before.