This is the fourth book in the Temeraire series. While I suppose the events in this book would mostly make sense on their own, I definitely recommend starting from the beginning.
This is quite an impressive story. Starting from a crisis that explains some events left naggingly unexplained at the end of Black Powder War, we dance through an impressive series of adventures to a point where things seem to have been safely concluded, only for another terrible thing to happen, and our heroes prove themselves truly heroic, but arguably foolish, with a cliffhanger that leaves this reader very frustrated that the next volume hasn't yet been published. I fear that Ms. Novik is starting to face the dilemma of writing an alternate history series -- the more important stuff starts happening in the alternate universe, the less one can rely on history to guide events and the more one must make up. Even accepting the starting premise that the Napoleonic world was pretty much the same, except that dragons were part of history too, things have to continue to evolve. She manages to bring major new elements into the overall story sufficiently successfully this time that it would be interesting to watch how long she can keep it going, even if the actual characters weren't so engaging that I'm absolutely desperate to find out how it works out.
I'm still too infatuated with Temeraire to trust my objectivity, but I think the overall craft is getting better. It still has imperfections, but I really enjoy the ride. 9 out of 10.
**** PLOT SUMMARY -- MASSIVE SPOILERS ****
At the beginning of the book we discover that a plague has infected most of England's dragons. They start coughing, it gets worse, and they die. It's highly contagious and none seem to recover. Temeraire and the ferals he brought with him are vital to bolster England's defenses, and they are working very hard, when a battle drops Temeraire into a quarantine covert. Laurence despairingly waits for Temeraire to get sick, but he doesn't. Soon the doctors realize that he actually had it -- he had a case of sniffles at the start of Throne of Jade -- but he recovered. So his wing, including a very sick Maximus, are bundled onto Allegiance and sent to retrace the route to see if they can reproduce the cure. At the last minute, Laurence is prevailed upon to take Erasmus, a free black preacher, and his family on board; they wish to do missionary work in Africa. This creates a great falling out with Riley, whose family are slavers, and makes things very tense on the journey. Laurence and most of the rest assume that the climate will do the trick, but the other dragons, though they do slightly better when they reach the tropics, are clearly not thriving. Eventually they acquire another of the huge smelly mushrooms like the one Temeraire tried on the first journey, and discover that it's the cure. Unfortunately, the mushrooms are terribly rare; eventually, they learn that the locals eradicate them because they're a threat to livestock. So Temeraire and the dragons that have gotten the cure -- Lily and two of the smaller ones -- head into the interior. They discover a cave where the mushrooms are growing, and send all of the dragons off loaded with all they can carry before they realize that the mushrooms in the cave were cultivated. And the owners show up to object. With dragons. Erasmus, the missionary, gets killed, but (slightly annoying deus ex machina) the dragon recognizes Mrs. Erasmus. The humans are taken prisoner and hauled roughly to an amazing human-dragon city at Victoria Falls, while Mrs. Erasmus manages to work out and explain that this was the tribe she was stolen from, and the dragon who captured them is her ancestor. It seems that the humans raise dragon eggs to believe that they are the reincarnation of worthy ancestors, and all believe that they actually are. Temeraire finds Laurence and the others in the prison, but he's captured. They're then all taken to a big gathering they fail to understand; it's sort of made to look like it's going to be some sort of ritual execution of Temeraire and/or the humans. One of the other dragons from the wing finds them and signals that the rescue will come shortly, and the rescue is a success. Only when they arrive at Cape Town, it is to find a huge force of African humans and dragons besieging the fort. The wing rescues the surviving British and they evacuate on Allegiance but the colony is destroyed. On the way north, they find that all the other slave ports have been destroyed as well. Laurence finds a couple of British slavers who expect him to rescue them and their wares; instead, he frees the slaves.
They triumphantly return to England, having discovered the cure, and just after they have the experience of flying over a bunch of strange new hills and realizing that the hills are the buried dragons that have been lost in the plague, they discover that the high command has send the plague over to France, freeing a captured, infected French dragon and sending it to a courier covert. France (and all of Europe)'s dragons will presumably die -- a great military windfall for England but a ghastly price to anyone who sees dragons as people. Temeraire and Laurence are both resolved to commit treason and give France the cure; they steal a tub of the mushrooms and fly to France. At first they are not well treated, but it gets a little better as the French realize that the cure is real and much better when de Guignes (the French ambassador to China in Throne of Jade) finds them. They meet with Lien, who is much more willing to be civil than Temeraire, and Napoleon himself, who is deeply grateful. But they will not turn their coats farther than they already have. Laurence insists on returning to England, though he is sure he will be executed; Temeraire insists on accompanying him. The book ends as they are in transit, leaving the reader to worry for months about what will happen to them and to reflect on the puzzle of whether it is really honor, or really some insidious form of insanity, that binds a good man to stay loyal to a country that has done something terrible, and is about to compound the moral outrage by executing the citizen who tried to stop it.