Leviathan follows the story of two teenagers in the early days of World War I. But this isn't your grandfather's (okay, maybe your great-great-grandfather's) World War I. Deryn Sharp and Prince Aleksander of Hohenberg live in an alternate version of reality in which the whole world is split into Darwinists and Clankers. Darwinists fabricate fantastic beasties using the theories of Charles Darwin, who in this world discovered DNA. Clankers, on the other hand, depend on their skill with machines.
When Alek's parents, Archduke Franz Ferdinand and his wife, Princess Sophie, are assassinated, Europe is plunged into war: Darwinists versus Clankers. Alek begins his own personal battle for survival as his parents' assassins come after him, and along with a few companions, Alek makes it from Austria-Hungary to Switzerland, where his protectors intend for him to wait out the war in hiding.
Meanwhile, Deryn Sharp is a girl who loves to fly, but girls can't be airmen. So Deryn disguises herself as a boy and earns a place as a midshipman on the British airship Leviathan, a massive ecosystem in the sky. As Britain declares war on Germany, the Leviathan is shot down, landing in Switzerland.
As German forces come after the hidden prince and the wounded airship, Deryn and Alek join forces to save themselves, their companions, and their secrets.