I know I usually say "I don't know how to even describe the plot" but in this case, it's really true. Not only is the plot unbelievably weird, so is the world that you find yourself in.
So, here's an attempt.
Basically, it's very, very far in the future. There's been huge advances in bio-engineering to the point where there are basically no more men, no one has babies anymore (they're all sort of grown like plants?), and Mars and the Matriarchy are the dominant political forms in the galaxy. However Nightshade (one of the planets in the far reaches of the solar system) is basically making a powerplay to disrupt the powers that be and change life as they know it.
Our main characters are Dreams-of-War, a warrior of Mars. She has special 'haunt-tech' armour (infused with the soul of someone else from the Eldritch realm) and is sent to Earth to guard Lunae or the hito-bashira, who was bred to stop the future devastation of the solar system. Lunae's talent is that she can bend time, moving backwards and forwards through time (sometimes taking people or things with her). However, she ages incredibly fast (in about a year she goes from born to an adult) and doesn't fully know how to control her powers. There is also the kappa (or nurse) who is part of a genetically altered form of human (amphibious) and considered an underclass on Earth. Finally, the last of the main characters are Yskatarina, an agent of Nightshade and her companion or Animus (also something genetically engineered that can fly and shoot napalm out of its face, basically).
So then there are the worlds themselves, where everything is genetically altered in radical ways. Dreams-of-War doesn't just have her armour, but has had enhancements to her fingers, teeth and mouth to be more deadly (also, um, down there, which is alluded to at one point). There's the excesseries (or scissor women) who are even further modified warriors that guard the Matriarch and do her bidding.
There are...genetic...rejects (I suppose) as well, that have lasted down the centuries but are referred to more like animals than humans (and some of them are, gasp, males. Then there's even weirder things, like the ground which contains people or souls, that can trap the unwary traveller. It's just utterly bizarre to have to try and explain. Just imagine bio-engineering used as weapons for centuries and the experiments gone wrong (or right) and that have been left to their own devices for centuries. LOTS OF WEIRD THINGS.
Because the world is so strange, it takes a while to get up to speed with the story. However, I think the author Liz Williams does it with good grace, so you're never going "what the fuck is going on" too often to give up on it. Eventually, it all kind of makes rough sense and you get right into the story. There's lots of weirdness happening, but the main plot is keep Lunae away from the assassins (or if you're the assassin, find Lunae).
Through the individual motivations of the characters, you find out what Lunae's destiny is and what will happen if she fails. As well, what 'haunt-tech really is and where it's come from. There's a series of other weird characters like Yskatarina's Aunt Elaki and Lunae's "Grandmothers" that put all the main characters in motion (and their underlying motivations are interesting and coherent as well).
My only criticism would be that some of the weird things like Nightshade's "Mission" (think like a cult's church), are talked about a lot but essentially are irrelevant to the plot. I think it's wrapped up in one sentence which didn't merit the inclusion in the book (I thought). As well as Essa, who shows up quite late in the book and is supposed to be a great help, does relatively little and then disappears.
Also, I think I have to read the ending again as I really don't understand what happened. The problem with having really indescribable parts of the plot, which then have to meet and do something indescribable, it just leaves you going "huh?"
But regardless, it was pretty good, if utterly alien and strange. It's a disordered universe with unbelieveable technology. It's an unrecognisable solar system with our own history now grown to mythology. Give it a whirl if you want something completely different than things you've read before. I'd say it's probably closest to China Miéville in descriptions and world building but with better pacing and an actual plot. You'll need a quiet hour to get into the world first, otherwise you could get very, very lost.