This is book number 3 in the Imperial Radch series.
Things seem to be under control for the soldier known as Breq-until a search of Atheok Station's slums turns up someone who shouldn't exist. Meanwhile, Breq's enemy arrives, the insane Anaander Mianaai. Breq could take her ship and flee, but that would leave everyone at Athoek in danger, so she forms a plan. The odds aren't good, but that's never stopped her before.