The premise is that the human race is suffering large numbers of losses from a mysterious extra-dimensional force that is destroying them in an attempt to stop all portal network travel. They cannot or won't communicate with humanity and mankind is not really doing much to try on their end. Laconian dictator, Duarte, has awakened from his coma but the creature that has emerged is not necessarily the man who went under.
The Rocinante crew is also on the run, a state they're familiar with, as they have managed to free Holden from Laconian imprisonment as well as get away with Theresa Duarte. Unfortunately, their knock-out blow against Laconia has failed. The Laconians are still the most powerful force in the human star systems and they are continuing to dominate humanity's thousand colonies.
Because Duarte is basically a ghost trying to become a god (and thus off camera for the majority of the book), the majority of the tension comes from Colonel Tanaka who is determined to hunt down the Rocinante crew. She is an exceptionally talented and skilled officer who is, nevertheless, completely loyal to the Laconian ideal. She develops a patholical hatred of James Holden during their first encounter and becomes their personal Boba Fett for the rest of the book.
I've never been a huge fan of the protomolecule elements of the story over the social critique and conflict among nations. In a very real way, the Goths and protomolecule elements have always been like the White Walkers in A Song of Ice and Fire for me. Which is to say they're a major part of the setting but not what I read it for. I would have much preferred continuing the war against Laconia and its embodying of humanity's worst impulses to authoritarianism and militarism but the story was never going to end there.
The story focuses a lot on the nature of the Goths, the nature of the protomolecule race, and also how humanity might have to evolve itself in order to use the protomolecule race's technology. Sadly, there's not much character development of the Goths and I was really hoping someone would try to communicate with them. The Expanse takes the view that aliens are going to be, well, alien, and the idea of being able to interact is something that requires absolutely breathtakingly weird circumstances. It requires several miracles to interact with the protomolecule machinery "spirits" and the Goths are an order of magnitude weirder than they are.
I would have appreciated a little more insight into the fates of several characters in the series, particularly Filip Inaros/Nagata but I feel like the book overall stuck to the landing. We get conclusive endings to a number of the characters and all the major plotlines are finished. One character ends up in a position I never thought he would and if he becomes the guiding principle for humanity, well, God help us all.