In an alternate World War I era, Europe is divided between Clankers who build mechanical war machines and Darwinists who create genetically-engineered living creatures. When Austrian Prince Aleksandar's parents are assassinated, he flees in a walking war machine while Deryn Sharp, a Scottish girl disguised as a boy, joins the British Air Service aboard the living airship Leviathan. Their fates collide when they must work together to survive in a world at war, challenging their beliefs about technology, friendship, and what makes someone a monster.