Leviathan takes place in an alternate world at the start of World War I. Darwinists are able to create a huge airship which is really an ecosystem, a “fabricated beastie.” Opposing the Darwinists are the Clankers who have incredible mechanical walking machines. The two main characters are fifteen year old Deryn and Alek.  Deryn is disguised as a boy so that she can volunteer for the British Navy as an airman. Alek is an Austrian prince trained in war. Alek and Deryn become allies.

Keith Thompson is the illustrator and his drawings add to the realism. The genetically engineered animals would be hard to comprehend without his drawings.

Westerfeld has written some masterful fight scenes for both Alek and Deryn.