The novel opens at the beginning of an alternative version of World War I: the Austro-Hungarians and Germans use Clankers, diesel-driven mecha loaded with guns and ammunition, while the British Darwinists employ genetically engineered animals as their weaponry.

Deryn, poses as a boy ("Dylan Sharp") and goes aloft with a Huxley (a fictional jellyfish-like creature that uses hydrogen to float) to prove her airworthiness.

The crew fights back and defeats the planes, but crash-land in Switzerland on a glacier where Aleksandar and Volger witness the crash from their refuge.

Alek's "family" comes to his rescue, and battle almost erupts between the two sides, but Deryn's quick thinking brings everyone together to talk under a flag of truce.

Realizing their differences are outweighed by their similarities, Alek offers a sizable chunk from their food storage so the ship can replenish its hydrogen supply and take off again.

As they travel back to the Leviathan, two German zeppelins appear and send out commandos to capture them, severely damaging the Stormwalker and making it impossible to repair.

The novel closes with the Leviathan continuing its flight towards Constantinople with Alek watching the mysterious eggs that will hatch into some unknown fabricated species.

"[7] It also gained a starred review from School Library Journal, who said it was "full of nonstop action" and that "this steampunk adventure is sure to become a classic.