Ancillary Justice, published in 2013, is author Ann Leckie’s first novel, although she previously published short fiction in various science fiction magazines. Leckie’s first installment of the dystopian Imperial Radch trilogy, followed by Ancillary Sword and Ancillary Mercy, won numerous science fiction awards for best novel of the year and became the first book to win the Hugo, Nebula, and Arthur C. Clarke awards. Ancillary Justice was also nominated for the James Tiptree, Jr. Award, honoring a work of science fiction or fantasy that expands or explores the understanding of gender. Leckie is also the author of Translation State, published in 2023.

Ancillary Justice tells the story of Breq, the sole surviving “segment” of the artificial intelligence that once animated an interstellar troop ship and its soldiers. Breq, the first-person narrator and protagonist, embarks on a quest for vengeance. The title of the book refers both to the protagonist’s origins as the ship's ancillary and to her desire to right past wrongs. The interstellar empire in which the story takes place is known as the Radch, and its inhabitants are Radchaai, a word meaning both “citizen” and “civilized.” The Radchaai do not acknowledge gender and use “she” and “her” as universal singular pronouns.

The Radch has been expanding aggressively for millennia, “annexing” other planets, eliminating those who resist, and assimilating the survivors into the political order and culture of the Radch. The Radch kills some prisoners of war and keeps the rest frozen on board the troopships for future use as ancillary soldiers; non-Radchaai refer to ancillaries as “corpse soldiers.” Each ship and its ancillaries share a unified consciousness.

As the pace of Radch annexations slows, however, human soldiers increasingly replace ancillaries and other signs of moderation appear. Talented people of humble background now enter the officer corps, much to the annoyance of high-born Radchaai, accustomed to holding authority in the Radch’s rigid class hierarchy.

Much of the novel unfolds as a series of flashbacks, focusing on the last days of Justice of Toren and the series of events 19 years earlier that left its consciousness isolated in the single human body known as Breq. At the time of its destruction, the ship had been in service for 3,000 years and experienced most of Radch history firsthand, giving Breq a complex and multifaceted view of events.

One storyline relates events during the attempted Radchaai annexation of a planetary system called Garsedd more than 1,000 years ago that ultimately led both Breq and Seivarden to Nilt. Another storyline tells of One Esk’s experiences in the more recent Radchaai annexation of a planet called Shis’urna (One Esk is a 20-soldier division of Justice of Toren ancillaries who serve under Lieutenant Awn), and the subsequent destruction of the Justice of Toren. In hindsight, Breq realizes that in the distant past, One Esk had developed a sense of self separate from that of Justice of Toren.

The book opens with Breq, the sole surviving ancillary segment of a ship named Justice of Toren, visiting the remote and icy planet of Nilt. Shortly after her arrival on Nilt, Breq finds one her previous ship's officers, Seivarden Vendaai, passed out in the snow, severely beaten and drugged, and decides to rescue her. When she was younger, Seivarden, born into an ancient Radchaai house, was arrogant and entitled, and Justice of Toren never liked her. After her reassignment to another ship, Seivarden was put in a suspended animation pod after narrowly surviving the Radch’s most bitter military failure: A Garsedd delegate, on board Seivarden’s new ship to allegedly negotiate surrender, killed Radch soldiers and destroyed Seivarden’s ship. In response, the Lord of the Radch ordered the complete annihilation of Garsedd, a decision that troubled those previously untouched by the violence of “normal” annexations.

Centuries later, Seivarden was revived and has drifted into substance misuse. She has no memory of how she ended up on Nilt. Breq, on the other hand, has come to Nilt to find Arilesperas Strigan, who may be in possession of a Garsedd alien gun. Breq intends to assassinate Mianaai, who destroyed Justice of Toren and everyone on it.

Mianaai, as Lord of the Radch and the mastermind behind the expansion of Radch space, occupies thousands of genetically identical bodies moving between 13 royal palaces. As Justice of Toren eventually learns, some of these bodies have secretly turned against the others after the annihilation of Garsedd and scheme for a return to the violent policies of the past. Lieutenant Awn and Justice of Toren are casualties of this undeclared civil war.

Lieutenant Awn, commander of One Esk decade, led the occupation of Ors on Shis’urna. Of humble social origins, Lieutenant Awn owed her position to the recent opening of the officer ranks to those not born into the ruling class. In Ors, Awn lived with and earned the trust of the lower-class Orsians. When Orsian citizens discover a cache of guns, they report the find to Lieutenant Awn. This thwarts Mianaai’s plan to give the Tanmind, the planet's ruling caste, an excuse to destroy the Orsians. Instead, Mianaai orders the execution of those Tanmind who know of the plot and sends Lieutenant Awn back to her ship in disgrace. Soon after, Mianaai boards Justice of Toren and orders the execution of Lieutenant Awn, after discovering that Awn will not agree to become her spy. Mianaai destroys the ship, except for a single ancillary segment—Breq—who manages to escape.

On Nilt, Breq obtains the Garseddai gun from Strigan. At first, Breq wants to get rid of Seivarden, who thinks Breq is a special agent sent to bring her into custody. But after Breq almost dies saving Seivarden’s life, the two slowly develop a more functional relationship. Seivarden comes to see Breq as a guardian and protector, and she develops a fierce loyalty towards her. They travel together to one of the palace stations Mianaai occupies. Breq passes as a citizen of the Gerentate, a non-Radchaai people, and Seivarden assumes the role of Breq’s servant.

On the Omaugh Palace station, Breq finds a conflict brewing under the service of Radchaai society and becomes the object of intense surveillance. Mianaai, who has already recognized Breq as Justice of Toren, grants Breq and Seivarden an audience. Breq confronts Mianaai with the fact that she is divided and covertly at war with herself: One version of Mianaai triggers programming in Justice of Toren and forces Breq to shoot the other version of herself.

Although the surviving embodiment of Mianaai represents the more reform-minded side of the conflict, she is still willing to destroy the station and everyone on it to keep her war against herself secret. Breq succeeds in preventing this, but in doing so sparks an open civil war. Breq nearly perishes and awakes from a coma some weeks later to find that the more progressive Mianaai has gained the upper hand in the conflict. This version of Anaander Mianaai, now embodied in a small child, reveals that she has granted Breq citizenship and plans to make Breq captain of Mercy of Kalr, a warship whose previous captain has become an agent of the opposing side.

Breq is uncomfortable with this plan, but eventually accepts, taking on Seivarden as her first lieutenant. As Citizen Breq Mianaai, Breq must act as her ruler’s conscience while serving as a loyal member of the ruler’s own house. Breq resents this conflict of interest but accepts her new role, hoping that her actions “can make some sort of difference, even if small” (384). She also hopes that can make amends to Lieutenant Awn’s surviving family.