In the fourth and final brilliant installment of Philip Reeve's award-winning quartet, after twenty years, the devastating world war between the predatory Traction Cities and their enemies is ending. Wren and her father Tom Natsworthy travel the Bird Roads in their airship, trying to forget Hester's betrayal. But in the ruined wreckage of the city of London they make a discovery that changes everything. And Hester faces an implacable foe with the means and the will to destroy the entire human race.