Operation Ludendorff: a Leviathan fanfic

It's 1915, the Great War is about to end, when word gets out about another German plan. Aleksandar Hohenberg and Deryn Sharp, both fresh from their latest mission for the Society, are tasked with destroying the Paris Gun, a machine capable of destroying a whole city.
Reminder: You won't understand a thing in this story unless you read Scott Westerfeld's Leviathan series.