The book follows the story of Livira, a young and brilliant orphan from the Dust, and Evar, one of five children who were separated from their time and raised together in an immense and inescapable library.
At the center of the city is a mysterious library containing the complete knowledge of ages before the Dust, when humanity could cure disease and reach into the stars.
After being orphaned by a sabber attack on her tiny village in the Dust, Livira is taken to Crath City by Malar, a swordsman who was among the men who rescued her.
Though she faces discrimination from the city residents, she is taken under the wing of Davris Yute, an important librarian who ensures she is educated by the library staff.
Evar, meanwhile, lives in an isolated chamber of the library, unable to leave or interact with anyone other than his adopted siblings, the automaton-like Assistant who has raised them, and the implacable Soldier who protects them.
Evar and his siblings were each taken from their own time while using the Mechanism, a device inside the library that lets people enter the world of whatever book they carry.
Though they share a longer conversation, Escapes begin attacking the Exchange, and Livira flees back to her own library while Evar jumps into a new one.
Crath City's king and political leadership call on the library to provide texts that dehumanize the sabbers and equate them to animals.
During her quest to understand the library's history and return to Evar, she also encounters the head librarian, a mysterious figure with some unknown connection to Yute.
Livira is a human, like the ones who slaughtered Clovis's family, while Evar is a canith, like the ones laying siege to Crath City, which in fact has passed back and forth between the two species for generations.
During the standoff, Yute explains that they are all part of a larger conflict taking place between forces that support and oppose the library.
At a critical moment, Mayland, the missing sibling, emerges from one of the portals and accuses the head librarian of taking the wrong side, executing her.