Hester reveals that she knows Shrike, who took care of her after her parents were killed by Valentine as they would not give up an Old Tech machine. Tom and Hester escape by stealing a hot-air balloon and drift over the Hunting Ground. At Airhaven, they are then attacked by a semi-robotic " Stalker" called Shrike, who was sent after them by the London Mayor Magnus Crome to kill them and bring their bodies to him. Tom and Hester manage to escape from Wreyland, meeting a friendly airship pilot called Anna Fang, who takes them in her airship the Jenny Haniver to the neutral flying city of Airhaven, where they can find passage to London. The pair eventually board a small town called Speedwell, where the owner Orme Wreyland attempts to harm them and schemes to sell the pair as slaves for profit at a trading cluster. Tom and Hester recover outside of London within the Hunting Ground, and after an argument start following the city's tracks to reboard it. When Tom informs Valentine of her name, the Historian pushes him down into the chute. She reveals a disfiguring scar on her face and claims Valentine caused it, before escaping the London police through a chute. One of Salthook's citizens, teenager Hester Shaw, attempts to assassinate Valentine, but Tom interferes and chases her. Tom incidentally meets the Head of the Guild of Historians, Thaddeus Valentine, along with his daughter, Katherine. Tom Natsworthy, a teenage Apprentice Historian, is sent to the " Gut" of London, where towns are stripped for resources, after he skips a chore.
The book starts with the traction city of London chasing and catching a small mining town called Salthook. The great town moves off after its quarry as events within the walls begin to take a sinister turn. The city has been lying low, skulking in the hills to avoid the bigger, faster, hungrier cities loose in the Great Hunting Ground. Europe, some of Asia, North Africa, Antarctica, and the Arctic are dominated by traction cities, whereas North America was so ravaged by the war that it is often identified as the " Dead Continent", and the rest of the world is the stronghold of the Anti-Traction League, which seeks to keep cities from moving and thus stop the intense consumption of the planet's remaining resources.
Because scientific progress has almost completely halted, " Old Tech" is highly prized and recovered by scavengers and archaeologists. Much technological and scientific knowledge was lost during the war. Although the planet has since become stable, Municipal Darwinism has spread to most of the world except for Asia and parts of Africa, with the primary location of many traction cities being the Great Hunting Ground. The technology rapidly spread, and evolved into what is known as " Municipal Darwinism", and the emergence of " traction cities".
To escape the instabilities, a Nomad leader called Nikola Quercus (known as god Nicholas Quirke by the time of the book) installed huge engines and wheels on London, and enabled it to dismantle (or eat) other cities for resources. Mortal Engines is set in a post-apocalyptic world ravaged by the Sixty Minute War, which caused mass geological upheaval.