In the last years of the Forerunner empire, chaos rules. The Flood--a horrifying shape-changing parasite--has arrived in force, aided by unexpected allies. Internal strife within the ecumene has desperately weakened Forerunner defenses. Too little, too late, the legal rate of Juridicals is only now investigating possible crimes by the Master Builder and others. Evidence-gathering agents known collectively as Catalog have been dispatched to collect testimony from the Librarian and both Didacts: the Ur-Didact, treacherously abandoned in a Flood-infested system, and the Bornstellar Didact, who accompanies the Librarian as she preserves specimens against the dire possibility of Halo extermination. Facing the imminent collapse of their civilization, the Librarian and the Ur-Didact reveal what they know about the relationship between the long-vanished Precursors and the Flood.