It's about a starship called Justice of Toren, a troopship that belongs to a human empire called the Radch. The bulk of its crew are ancillaries - human bodies with cybernetic implants that replace the body's personality with that of that of the ship.
The story shifts between two points 19 years apart. In the first Justice of Toren is involved in the annexation of a world into the Radch empire, which ends in tragedy. The second focuses on a single Ancillary, the only survivor of the ship's destruction on a mission of revenge.
While the plot is kinda standard SF, Leckie's writing elevates the book in fantastic ways, notably through the use of a few nice tricks.
First, the entire book is written in first person with a clear narrative flow, regardless of whether the chapter is written from the perspective of Justice of Toren the ship, or Breq the surviving Ancillary. They're the same person regardless of their physicality.
Second, the chapters set in the past timeframe attempt to capture how it feels to be a distributed consciousness. Justice of Toren talks about what's happening on the ship, on the planet, what different bodies are doing at the same time. It really helped to sell the ship's perspective, and gave the present timeframe chapters a feeling of loneliness thanks to their absence.
Finally, the book only uses female pronouns, explaining that the Radch language doesn't use gendered pronouns. There are a handful of moments where a characters sex is noted "She was obviously male" is a line early in the book, for example. This feels weird to begin with, and helps make the Radch feel alien while still biologically human.
It was the first novel to win the Hugo, Nebula and Arthur C. Clarke awards, blew me away when I read it.