In a futuristic steampunk world, humanity has rebuilt technology from scratch following a cataclysmic event called the Sixty Minute War. Cities like London have giant tracks that let them move at speeds of up to 100 miles an hour, catching smaller towns and stripping them for resources in a system called Municipal Darwinism.
Tom Natsworthy is a 15-year-old orphan and an apprentice at the Museum of Natural History in London. He idolizes the head Historian Thaddeus Valentine. One day a mysterious scavenger girl from the muddy area outside London boards the city and attempts to assassinate Valentine. Tom stops her at the last minute, chasing her when she flees. The girl has a large scar on her face and reveals that her name is Hester Shaw. Before jumping down a trash chute, Hester warns that Valentine isn’t who he seems to be. When Tom brings up Hester Shaw to Valentine, he pushes Tom down the same trash chute, attempting to kill him.
Tom survives the fall and finds himself outside London, in the mud next to Hester. She curses him for stopping her assassination attempt, but since they’re both trying to make it back to London, they agree to travel together. Meanwhile, back in London, Valentine’s daughter, Katherine, slowly begins to learn that her admired father may have a dark side.
Tom and Hester struggle to catch up with London, traveling on foot across the muddy area known as the Hunting Ground. After some initial setbacks, they meet a helpful airship pilot named Miss Anna Fang who is willing to take them to London. Their journey gets interrupted, however, when Shrike catches up with them. Shrike is a Resurrected Man, meaning he is a soldier from a previous era whose dead human brain powers a robotic body. He took in Hester for a while after Hester’s parents died, but now he seems to be a ruthless hunter. Lord Mayor Magnus Crome of London has sent him to kill both Tom and Hester.
To escape Shrike, Tom and Hester steal a hot air balloon while Miss Fang stays back to distract him. They can’t evade him for long, however, because he has heat-seeking vision and can track Hester’s scent. When he catches up with them, Shrike reveals that, while he does intend to kill Hester, Crome has promised Shrike that he will allow Hester to become Resurrected like him. Although Hester tentatively agrees to this, Tom intervenes, killing Shrike with a sword.
Meanwhile, back in London, Crome makes plans to use a super-powered weapon called MEDUSA that can obliterate whole cities. He wants to use it on the Shield-Wall, a giant barrier that protects the people in the Anti-Traction League from predatory moving Traction Cities like London, so that London can expand its hunting. Crome sends Valentine out in his airship on a secret mission.
After Shrike’s death, Tom and Hester reunite with Miss Fang. She takes them in her airship to a settlement behind the Shield-Wall. Tom realizes that the stories he heard in London are wrong and that the people behind the Shield-Wall aren’t evil. While Tom is exploring the settlement, he notices Valentine in disguise. Tom tries to catch him but can’t stop Valentine from sabotaging the settlement’s whole fleet of airships. Miss Fang confronts Valentine in the hangar and he tricks her during a sword fight, killing her.
Tom and Hester head back to London in Miss Fang’s old airship to try to stop Crome from using MEDUSA. Tom drops Hester off so that she can finally get her revenge on Valentine. At the same time, Katherine, horrified by MEDUSA’s destructive power, collaborates with some members of London’s Guild of Historians to attempt to sabotage it.
While Tom is flying around above London, waiting to pick up Hester, another airship attacks his. Tom fires back, and the ship falls down to London, starting a fire. Meanwhile, Crome’s guards catch Hester and bring her to Valentine. On her way to sabotage MEDUSA, Katherine sees Valentine and Hester. Before Valentine can kill Hester with his sword, Katherine leaps in the way, saving Hester but getting mortally wounded herself. As Katherine falls, she hits the controls from MEDUSA, causing it to malfunction. With no target entered, MEDUSA starts firing at London itself. Tom comes back in the airship and picks Hester up just in time to escape before London blows up. The two fly off to get their ship repaired, unsure what to do next but happy to at least have each other.
