The premise is that the solar system has been conquered by the Laconian Empire. They are a colony established by Mars deserters that have gained access to a shipyards owned by the Protomolecule race. Easily crushing the rest of humanity, they have established a British-style colonial empire that rules reasonably soft but still with absolute authority. Holden has been captured and the rest of the team is struggling to figure out a way to fight a seemingly invincible foe. Worse, Laconia continues to poke the bear of the race that destroyed the Protomolecule race. They believe they can deal with them as equals despite the fact they are seemingly possessed of far greater technology.
This is probably the best storyline for Naomi in the series as it gives her the opportunity to become the leader of the revolution against the Laconians. Too often, Holden was given the role of leader and it's good to see Naomi show herself off as a practical alternative. Holden also becomes a "evil chancellor" working against Duarte from behind the scenes while ostensibly his pet prisoner. Amos, Bobbie, and Alex all get great storylines and this is probably one of the best books for character development in the series.
I don't like what happened with Amos but that's just how these things go. We also lose a couple of beloved characters but lethality was always a part of the series. Indeed, the first sentence of the novel is a tragic loss of someone who many fans considered to be their favorite character. The characters are aging, though, and soon will probably be too old to continue their adventures much longer. The time skip between books is one of the more controversial decisions by James S.A. Corey. The heroes are only as good as their villains and Duarte is probably the best villain in the series.
This is due to the fact they manage to make him someone who is intelligent and charismatic enough to believe he could command all of the Laconian military but also dumb enough that he thinks picking a fight with Cthulhu is a good idea. It's amazing how someone so smart can end up being so stupid. Hopefully, his daughter will prove to be a better leader to the Laconians if humanity manages to survive.
In conclusion, this is among the best of the Expanse books. It's come a long way from the conflict between Belters, Earthers, and Martians. That's not entirely a good thing but I love this final leg of the adventure.