The novel focuses on Breq, the AI of a vast starship which has been destroyed. Breq's intelligence survives in one of its crew, an animated human corpse. Breq becomes an important player in the fate of the massive interstellar empire known as the Imperial Radch. The novel was well-received on release, spawning two (somewhat less-accomplished) sequels, Ancillary Sword and Ancillary Mercy, and winning the Hugo Award for Best Novel.
Knightly notes that in preparation for her role she's reading books about dictators and conquest, suggesting she might be playing Anaander Mianaai, the Lord of the Radch, rather than Breq herself, but that remains unconfirmed.
Reportedly, Ancillary Justice will start filming later this year.