Alek and Deryn, I mean, Dylan are back for the second installment of the Leviathan Trilogy. Picking up directly after the close of Leviathan, Behemoth finds Alek and Dylan racing toward Istanbul in hopes of keeping the Ottoman Empire out of the conflict between the Clanker Germans and the Darwinist British Empire. Neither teen knows all of the other's secrets, but they have become friends. Their friendship is tested as Alek and part of his contingent escape the airship Leviathan and become caught up in the rising rebellion against the Ottoman sultan. In addition, Dylan struggles with her desire to tell Alek her true gender as she begins hoping he will see her as more than a friend.