Hester Shaw and Tom Natsworthy have been flying their airship, the Jenny Haniver, for two years, following the destruction of London, one of the post-apocalyptic traction cities roaming the world on giant wheels. They are happily dating each other, and their ship is docked at the trade port known as Airhaven. The traction city of Arkangel is nearby, offering finder’s fees or “predator’s gold” to any air trader who can tell them the location of smaller traction cities that might be hiding from them. This is part of the practice known as Municipal Darwinism where strong cities take over weak cities, dismantling them into usable parts and fuel.
At Airhaven, Hester and Tom meet Professor Pennyroyal, an adventurer who writes books about his travels and barters a passage home on their ship. He tells Tom about America, known as the Dead Continent, a place supposedly decimated by the Sixty Minute War 1,000 years prior. Pennyroyal says he found a settlement of natives living in a green part of America, proving that the continent isn’t dead after all.
Gunships from the Green Storm, a fanatical branch of the Anti-Traction League, attack the Jenny Haniver because they want the plane that once belonged to Anna Fang, a spy and leader in the Anti-Traction movement. The Jenny Haniver escapes with severe damage and is blown north into the Ice Wastes.
In the Ice Wastes, they land on the traction city of Anchorage, which is bound for America. Margravine Freya Rasmussen is excited to have professor Pennyroyal aboard, because she is a fan of his books and has made the decision to travel to America partially based on his accounts of finding life there.
Meanwhile, in the sealed underwater town of Grimsby, a man called Uncle keeps surveillance on Anchorage through secret microphones and cameras. Uncle trains children to become master burglars, and his troop of thieves is called the Lost Boys. Uncle sends 14-year-old Caul to burgle Anchorage.
Caul’s ship, the Screw Worm, is docked under Anchorage, and he sets up cameras all over the city to keep track of citizens’ whereabouts to help him avoid detection as he steals things. As Caul watches peoples' lives through his cameras, he becomes emotionally attached to Freya, Hester and Tom.
Tom grows increasingly fond of Anchorage and of Freya. Hester is initially jealous and gets murderously angry when she sees Tom and Freya kissing. Hester starts the recently repaired Jenny Haniver and prepares to fly away without Tom. Caul watches these events through the cameras and goes to warn Tom, despite the risk it poses to his own burglary mission.
Tom is too late to stop Hester, who flies to the giant predator city of Arkangel and sells them the location of Anchorage on the condition that they will give Tom to her when they eat the city. Unfortunately for Hester, a bounty hunter captures her and takes her and the Jenny Haniver to the Green Storm headquarters at Rogue’s Roost, an island near Greenland.
The Green Storm’s commander is Sathya, Anna Fang’s underling, who Hester met two years prior in Batmunkh Gompa. Sathya blames Hester for Anna’s death and reveals that Hester’s real father was Thaddeus Valentine. Hester is crushed by that news and equally disgusted to discover that Sathya has brought Anna Fang back as a Stalker, a mechanized corpse without feelings or memories. Stalker Fang angrily insists that it is not Anna Fang and tries to kill Sathya when she visits.
Tom confronts professor Pennyroyal about his increasingly hard-to-believe historical tales and discovers that Pennyroyal never went to America, but rather invented stories to sell books. Meanwhile, Caul receives notice that he and the other boys on the Screw Worm are to kidnap Tom and bring him back to Grimsby.
Once at Grimbsy, Tom meets the Lost Boys’ master, Uncle. Uncle tells Tom that he wants to help Tom rescue Hester from the Green Storm. As a thief, Uncle doesn’t like the Green Storm’s impenetrable security and growing power.
In the Rogues’ Roost, Hester tries to convince Sathya not to use her growing army of Stalkers to attack all traction cities. The Screw Worm drops Tom off at Rogues’ Roost, where he is captured almost immediately.
The Lost Boys use his capture as a way to distract the guards while they send camera-spiders and bombs into the city. Uncle plans to blow up Rogues’ Roost and burgle the remains of the city. Caul is horrified to learn that Uncle intends to let the Green Storm kill Tom, so he detonates all the bombs prematurely.
While the Green Storm agents are distracted by the bombs exploding at their base, Tom finds Hester, and they both run to the Jenny Haniver. Stalker Fang meets them, and when Sathya tries to kill Tom, Stalker Fang cuts off Sathya’s hand. Tom and Hester escape, and Stalker Fang orders Sathya to attack Shan Guo so the Green Storm can use the Anti-Traction League’s resources to destroy all the world’s traction cities.
In Grimsby, Caul is sentenced to slow death by suffocation, as punishment for spoiling the mission in order to save Tom. Uncle tells Caul that love and compassion are weaknesses. Uncle himself once lived in Arkangel and fell in love with a slave he owned, who happened to be Anna Fang.
Anna secretly built an airship and escaped from Arkangel and from Uncle, which made him resolve to monitor everyone under his control and to never love anyone again. He had hoped to steal Stalker Fang and make it his slave as revenge, but since Caul ruined that plan, he intends to kill Caul. A younger boy named Gargle rescues Caul, who takes the Screw Worm to find Anchorage.
Freya discovers that professor Pennyroyal lied about going to America, but she has no time to change course because Arkangel is now pursuing them. Led by a man called Masgard, an airship of Huntsmen from Arkangel lands on Anchorage, attacking it just as Hester and Tom arrive in the Jenny Haniver to aid their friends.
Hester is motivated to help because of her intense guilt, since she never told Tom that she sold Anchorage’s location to Arkangel because of her own jealousy. Hester also feels like she was born to commit casual acts of violence because she’s Valentine’s daughter, and Valentine was a man with very little conscience.
Meanwhile, Freya learns from Masgard that it was Hester who betrayed her city, but she accepts Hester’s help in fighting Masgard. The citizens of Anchorage, led by Hester, defeat all of Masgard’s men. Hester kills Masgard with a sword as he begs for his life.
Professor Pennyroyal attempts to steal the Jenny Haniver, and he shoots Tom in the chest with a pistol when Tom attempts to stop him. Pennyroyal leaves in the Jenny Haniver, and Tom is left bleeding. Anchorage is still running from Arkangel when they hit a patch of thin ice. Arkangel gets stuck in the broken ice, and all its people evacuate, leaving it an empty, dead city.
Safe from attack, Anchorage drifts without direction on a large ice floe until the Screw Worm pops out from under the ice to meet them. Caul comes aboard Anchorage and gives Freya some ancient maps of America that he stole from Uncle. The whole city is filled with new joy and purpose, and they propel their city across the icy water until they reach a safe landing in America, which is beautiful and full of living things. Tom recovers from his bullet wound, and Hester discovers that she is pregnant with Tom’s child.