The great Traction City lumbers after a small town, eager to strip its prey of all assets and move on. Resources on the Great Hunting Ground that once was Europe are so limited that mobile cities must consume one another to survive, a practice known as Municipal Darwinism.

Tom, an apprentice in the Guild of Historians, saves his hero, Head Historian Thaddeus Valentine, from a murder attempt by the mysterious Hester Shaw -- only to find himself thrown from the city and stranded with Hester in the Out Country. As they struggle to follow the tracks of the city, the sinister plans of London's leaders begin to unfold ...

Tom and his daughter, Wren, aren't the only people interested in London. The desperate armies of the Traction Cities and the Green Storm are also closing in, certain that whatever is taking shape within the city holds the key to victory in their never-ending war.

But it may be too late. Even as Tom and Wren hurry to uncover the mystery of London, Hester Shaw—estranged from her husband and her daughter—tracks the resurrected Stalker Fang, who has found another way to end the war and all life on the planet once and for all.