This story takes place far in the future, in the Hexarchate a human empire divided into 7 factions - oh no 6 ... one turned out to be heretics and was annihilated a while back. Each faction is equivalent with a choice of profession. The Kel for example are the soldiers who guided by their formation instinct are super-humanly loyal. The Shuos are the spies, the Nurai the mathematicians and so on.

The defining element of the world however is the so-called high calendar which provides the empire with "exotic" technologies. Now here's the part that nearly made me put down the book at the start. Because the whole calendar thing is never quite explained, it is so self-evident for the people living in this world, that they never bother to explain it to the reader. In this aspect of "not explaining a thing and hoping the reader will still make sense of the world" it reminded me a lot of [b: Ancillary Justice|17333324|Ancillary Justice (Imperial Radch #1)|Ann Leckie|https://images.gr-assets.com/books/1397215917s/17333324.jpg|24064628] ... ultimately in a good way. At the end of the book I seem to understand a little bit more about what this is. Here's my take (Note: this is not a plot spoiler more of a world-building spoiler:) in this universe beyond our standard physics which is called the "invariant" there appear to be several mutually exclusive branches of physics and the high-calendar defines which branch is currently active in a given region of space. The calendar itself seems to depend on a mathematically guided system of concentrated belief via so-called "remembrances" that the citizens of the empire are forced to observe. So it is a case of weird science religion. In a fantasy novel I'd call the exotics magic ...

The main protagonist is Cheris, a Kel officer, who in her last battle used heretical formations to take an objective. As her squad is taken away for re-education, she is given a chance to redeem herself. Heretics have taken an important nexus fortress and she has to propose a plan on how to retake the fortress.

Once I got used to the generous helping of math terminology ("if you find a topological solution let me know") and ignored any possible deeper meanings, once I stopped thinking this might end up being some kind of matrix clone, and once I accepted that I wouldn't immediately understand the high calendar ... this book turned out to have delightful protagonists yes, both Cheris and Jedao right from the start and an incredibly intricate and interesting plot. I could not put the book down, and look forward very much to reading the sequel.

The end was good, a little surprising, and yet I still grieve for Jedao and Cheris, neither of which can be who they were now. Their interaction, which I enjoyed very much, is sure to change for the next book.