It is the cusp of World War I. The Austro-Hungarians and Germans have their Clankers, steam-driven iron machines loaded with guns and ammunition. The British Darwinists employ genetically fabricated animals as their weaponry. Their Leviathan is a whale airship, and the most masterful beast in the British fleet.

Aleksandar Ferdinand, a Clanker, and Deryn Sharp, a Darwinist, are on opposite sides of the war. But their paths cross in the most unexpected way, taking them both aboard the Leviathan on a fantastical, around-the-world adventure....One that will change both their lives...

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 will need great skill, new allies, and brave hearts...