It is the cusp of World War I, and all the European powers are arming up. The Austro-Hungarians and Germans have their Clankers, steam-driven iron machines loaded with guns and ammunition. The British Darwinists employ fabricated animals as their weaponry. The Leviathan is a living airship, the most formidable airbeast in the skies of Europe.

The behemoth is the fiercest creature in the British navy. It can swallow enemy battleships with one bite. The Darwinists will need it, now that they are at war with the Clanker powers. Deryn is a girl posing as a boy in the British Air Service, and Alek is the heir to an empire posing as a commoner. Finally together aboard the airship Leviathan, they hope to bring the war to a halt. But when disaster strikes the Leviathan's peacekeeping mission, they find themselves alone and hunted in enemy territory.

Alek and Deryn are on the last leg of their round-the-world quest to end World War I, reclaim Alek’s throne as Prince of Austria, and finally fall in love. The first two objectives are complicated by the fact that their ship, the Leviathan, continues to detour farther away from the heart of the war (and crown). And the love thing would be a lot easier if Alek knew Deryn was a girl. (She has to pose as a boy in order to serve in the British Airforce.) And if they weren’t technically enemies.

The tension thickens as the Leviathan steams toward New York City with a homicidal lunatic on board....