Philip Reeve's Mortal Engines is set in a post-apocalyptic world in which the so-called "Sixty Minute War" has ravaged the Earth. As a result of this upheaval, survivors have moved to cities on wheels, some of which share the names of the cities which preceded them (like London).
Mortal Engines begins with a battle scene between two of those cities—London and a small mining community called Salthook. The two cities are battling so that London can subsume Salthook's resources, a task which is typically completed by London residents being punished. At the start of Reeve's novel, that is young Tom Natsworthy, who was assigned to strip Salthook of its resources after he skipped a chore that he was assigned.
As Tom begins to carry out his duties, he comes face-to-face with a teenager who lives in Salthook. Her name is Hester Shaw and her mission in life is to kill Thaddeus Valentine, who is one of the most powerful people in London and who she has a history with. When Valentine learns of Shaw's presence in London, he pushes Tom and Hester out onto the desolate and largely inhospitable ground beneath them.
Initially, the duo is at odds. They don't have much in common and have different goals for their lives. Hester is set on revenge; Tom just wants to get back to London and wants to return to his life. Eventually, the two bond after they decide that they can accomplish their goals only with each other's help.
On their way back to London, the two encounter many challenges. For instance, they are forced to deal with the owner and operator of a small city who captures them in hopes of selling them into slavery. They are also attacked by a seemingly omnipotent cyborg focused on destroying them, as well as scout ships from London itself and violence from those who survived the war.
Eventually, Tom and Hester meet agents from the Anti-Traction League, whose focus is to stop larger cities from consuming smaller ones. To that end, they work with the Anti-Traction League to devise a plan to stop London, which they are eventually able to do. London is mostly obliterated by Tom, Hester, and the Anti-Traction League's attack, and Valentine is killed, leaving Hester and Tom as the city's only survivors.
