Agents for peace are bringing an end to the devastating conflict between the roaming Traction Cities & their fanatical enemies while Wren & her father Tom travel the Bird Roads in their airship trying to forget that Hester has betrayed them. In the ruined wreckage of the city of London they make a discovery that will change the world while Hester must face an implacable foe with the means & the will to destroy the entire human race. The fourth spectacular instalment of the award-winning quartet.