This is the sequel to Leviathan. Or rather, the continuation of a book chopped in thirds. I don't think it -- or this review -- will make much sense to anyone who hasn't read that.
Deryn/Dylan and Alek are taking advantage of their flying over the Mediterranean when spotting two German ironclads interrupts. Alek has qualms about helping Darwinists against Clankers, but tells his man to keep the Clanker engines, jury-rigged into the Leviathan going. And it's a good thing, because one of the Austrians recognizes that the Germans have got into operation the Telsa Cannon, and starts to get to the airship to reverse. Technically that's mutiny but since the cannon unleashes a furious bolt of lightning at them -- Deryn barely manages to rescue the other midshipman -- that gets more or less overlooked as they plug onward to Istanbul.
In Istanbul, they find out what those two ironclads were doing. They also get involved with intrigue, anarchists, a sultan with a clanker automaton, perspicuous loris being perspicuous, a doomed romance, attempts to flee, a very expensive hotel, an American reporter, and golems.