In the Traction City of Peripatetiapolis, Tom Natsworthy discovers that the bullet that Nimrod Pennyroyal shot him with in "Predator's Gold" has damaged his heart and he has not long to live, though he doesn't tell his daughter Wren. Tom soon discovers a woman that resembles Clytie Potts, a London Apprentice Historian that he knew. When questioned, she tells him that her name is Cruwys Morchard and that she is transporting Old-Tech devices. Unconvinced, Tom and Wren decide to pursue her, travelling on the "Jenny Haniver" to Murnau, a "Traktionstadtsgesellschaft" city, where they meet Wolf von Kobold, the son of the Mayor of Murnau and commander of the traction city Harrowbarrow. Wolf believes that the survivors of MEDUSA may be still inside London, and the three agree to make an expedition to London.