Alek and Deryn/Dylan begin the story en route to Istanbul where the scientist/spy granddaughter of Charles Darwin has eggs to present to the Sultan to sway the Ottoman Empire from supporting the Clankers. Remember that in this alternate history, the world is divided into Clankers – cultures using mechanical power and walking tanks and the like – and the Darwinists – cultures who bioengineer their tools. Germany and the Austro-Hungarian Empire are Clankers. Britain and Russia are Darwinist. America is an odd hybrid that no one really wants involved in Europe’s conflict.
The story has exciting air battles, spy/sabotage escapades, gender-swapped shenanigans (Alek and almost everyone else is unaware that Dylan is a girl posing as a boy to be able to be in the Air Service), revolutionaries, unconventional weaponry, Tesla lightning cannons and of course a giant sea monstrosity that might be able to keep the Ottomans out of the war.
This series is something I’d highly recommend.