The book starts with the traction city of London chasing and catching a small mining town called Salthook. Tom Natsworthy, a teenage Apprentice Historian, is sent to the "Gut" of London, where towns are stripped for resources, after he skips a chore. Tom incidentally meets the Head of the Guild of Historians, Thaddeus Valentine, along with his daughter, Katherine. One of Salthook's residents, teenager Hester Shaw, attempts to assassinate Valentine, but Tom interferes and chases her. She reveals a disfiguring scar on her face and claims Valentine caused it, before escaping the London police through a chute. When Tom informs Valentine of her name, Valentine pushes him down into the chute. Tom and Hester recover outside of London within the Hunting Ground and, after an argument, they start following the city's tracks to reboard it.