The Leviathan arrives in Constantinople, a city where Clanker culture and Darwinst principles intersect in the most intriguing ways. Dr Barlow and Deryn deliver their precious cargo to the Sultan, but their peace-keeping mission goes unexpectedly - and disastrously - awry. Now the only way to save themselves in this hostile, politically-charged city is for Dr Barlow to offer up the thing that matters most: the air ship. Alek escapes from his prison camp and goes on the run with his men and the loris while Count Volger stays behind to fend-off the pursuit, forcing Alek to take on new responsibilities.

Meanwhile a secret mission lands Deryn in serious danger...and leads both teens to re-evaluate their precarious situations in the world.