Emerging from its hiding place in the hills, the great Traction City of London chases one terrified little town across the wastelands. If it cannot overpower smaller, slower prey, the city will come to a standstill and risk being taken over by another. In the attack, Tom Natsworthy, Apprentice Historian to the London Museum, is flung from its speeding superstructure into the barren wasteland of Out-Country. His only companion is Hester Shaw, a murderous, scar-faced girl who does not particularly want Tom's company. But if they are to make it back to London before Stalkers or hungry cities get them first, they will need to help each other, and fast. London is planning something atrocious, and the future of the world could be at stake.