Mortal Engines Book Summary

The events of Mortal Engines, a post-apocalyptic, steampunk science-fiction novel by Philip Reeve, published in 2001, unfold in a post-war, dystopian world plagued with environmental degradation where cities have transformed into metropolises on giant wheels following the devastating Sixty Minute War. One such is the traction city of London, which can travel up to 100 miles an hour, engulfing, looting, and capturing smaller settlements, all in the garb of a system they call "Municipal Darwinism." The two distinct types of societies share a predator-prey relationship.

The protagonists of the novel, Tom Natsworthy, the orphaned fifteen-year-old apprentice at the Museum of Natural History in London, and Hester Shaw, the outsider girl with a scarred face, assigned as an assassin to kill the head historian Thaddeus Valentine, unite in strange circumstances creating a unique intersection of the two societies. They meet when Hester sneakily boards London to accomplish her task, and Tom catches her, jeopardizing her plan. A chase ensues, and Hester flees down a trash chute after informing Tom of Valentine's questionable intentions. Valentine learns about the incident and throws Tom down the chute in a futile attempt to kill him.

Now outside, beyond his secured life in the city, in the unforgiving environment of the muddy lands called the "Hunting Ground," Tom meets Hester once again. After arguments, the two decide to stay together as they struggle to make their way back on foot. After dodging a few perilous hurdles, they meet Miss Anna Fang, an airship pilot who agrees to take them back to London.  

Their quest is disrupted by Shrike, a hunter cyborg with advanced tracking systems and the body of a robot controlled by the brain of a dead soldier, sent by Mayor Magnus Crome to kill them. They attempt to evade Shrike in a hot air balloon as Fang fights to distract him. Shrike had once sheltered Hester after her parents' death. He now wants to kill her because Crome has promised him a similar resurrection for Hester. When he tracks her down and reveals that, Hester agrees to accept her fate before Tom pushes a sword into Shrike's cyborg body, killing him.

After this, they meet Fang and travel to the Shield-Wall, a protective barrier that hides the Anti-Traction League community from predators like London. He learns that the narrative that he has been fed during his life aboard London, about people beyond the wall being evil, was false. 

In London, Magnus Crome plans to destroy the Shield-Wall with MEDUSA, a powerful weapon of mass destruction that can annihilate entire cities. He sends Valentine on a secret mission beyond the wall, in disguise, to infiltrate, spy, and compromise their fleet of airships. Tom identifies him but fails to stop him. Valentine kills Fang during the ongoing chaos. Tom and Hester take Fang's airship to fly back to London to prevent Crome from using MEDUSA to destroy the settlement. On reaching, he drops Hester to fight Valentine.

Valentine's daughter Katherine, who is now aware of her father's dubious ways, gathers a few members from London's Guild of Historians to prevent the use of MEDUSA. Meanwhile, Tom hovers over London to extract Hester, is attacked, and fires back till the attacker's airship falls on London, starting a fire. Amid this chaos, Katherine heads to stop MEDUSA when she finds her father in battle with Hester. She interferes to stop him from killing Hester with a sword but finds herself mortally injured. She manages to mess up the control unit of MEDUSA before dying. The malfunctioning MEDUSA blows up London before Tom rescues Hester from the fallen city and flies off with her into the unknown.