Ancillary Justice is a space opera set thousands of years in the future, where the principal power in human space is the expansionist Radch empire. The empire uses space ships controlled by AIs, who control human bodies ("ancillaries") to use as soldiers. The Radchaai do not distinguish people by gender, which Leckie conveys by using "she" pronouns for everybody, and by having the Radchaai main character guess, frequently incorrectly, when she has to use languages with gender-specific pronouns.
The narrative begins nearly twenty years after the disappearance of a Radch starship, the Justice of Toren, when the sole surviving ancillary (and a fragment of the Justice of Toren's consciousness), Breq, encounters an officer, Seivarden, who had been a lieutenant on the Justice of Toren 1,000 years earlier. The two are on an ice planet, and Seivarden is in precarious condition. The plot switches between two strands: Breq's "present-day" quest for justice for the Justice of Toren's destruction and flashbacks from 19 years earlier when the Justice of Toren was in orbit around the planet of Shis'urna, which was being formally brought into the Radchaai empire. The reader eventually finds out that the Justice of Toren's destruction was the result of a covert war between two opposed strands of consciousness of the Lord of the Radch, Anaander Mianaai, who uses multiple synchronized bodies to rule her far-flung empire. At the end of the novel, Breq associates herself with the more pacifistic aspect of Anaander Mianaai while waiting for an opportunity to exact her revenge.
