Two opposing forces are on the brink of war. The Clankers - who put their faith in machinery - and the Darwinists - who have begun evolving living creatures into tools. Prince Aleksandar, the would-be heir to the Austro-Hungarian throne, comes from a family of Clankers, and travels the country in a walker, a heavily-fortified tank on legs. Meanwhile Deryn Sharp, a girl disguised as a boy, works for the British Empire, crewing the ultimate flying machine: an airship made of living animals. Now, as Alek flees from his own people, and Deryn crash-lands in enemy territory, their lives are about to collide...