Ancillary Mercy is the third book in the trilogy that began with Ancillary Justice.
For a moment, things seemed to be under control for Breq, the soldier who used to be a warship. Then a search of Athoek Station's slums turns up someone who shouldn't exist and a messenger from the mysterious Presger empire arrives, as does Breq's old enemy, the divided, heavily armed, and possibly insane Anaander Mianaai—ruler of an empire at war with itself.
Breq could flee with her ship and crew, but that would leave the people of Athoek in terrible danger. Breq has a desperate plan. The odds aren't good, but that's never stopped her before.