The Great Traction City of London was raided by Anti-Tractionist League forces. The city, made mobile over hundreds of years after the Sixty Minute War via Municipal Darwinism, a philosophy involved the consumption of smaller towns for resources, had been heading down towards Shan Guo in an attempt to make it the new Hunting Ground. Standing in its way was the Shield Wall, a structure that bordered Shan Guo, a key League nation. Bringing the wall down was part of Thaddeus Valentine's plan, so he deployed an ancient Old-Tech weapon, MEDUSA, of which he retrieved years prior, taking the life of Pandora Shaw in the process, to destroy the Wall.
The raid was a success, but not without the loss of Nils Lindstrom, Yasmina Rashid, Khora. Anna Fang managed to infiltrate St. Paul's Catheral in an attempt to take MEDUSA down, only to face Valentine in a duel which resulted in her death. Hester Shaw managed to slip into and disabled the weapon with the crash drive, Anna saw this outcome before falling to her death.
As Valentine left the Catheral, he was confronted by his daughter Katherine, who begged him to stop his evil ways. He shrugged off, focused on the objective of bringing down the Wall. Meanwhile, he had his henchmen kill the city's control crew and put the city on full throttle on a collision course with the Wall. With Katherine's assistance, Tom flew the Jenny Haniver inside to destroy London's engine. Hester caught up with and engaged Valentine aboard his airship, where he revealed that he was her father. Tom rescued Hester and shot down Valentine's ship, which was crushed by London's slowing tracks, killing Valentine. The surviving Londoners, now led by Katherine, make peace with the Anti-Tractionists led by Governor Kwan.
Human archeologists Pandora Shaw and Thaddeus Valentine found a weapon. She knew Valentine wanted for his own dark purposes and they fought over it. During the fight, Valentine stabbed Pandora in front of her daughter, Hester, and slashed Hester's face when she tried to save her mother. Before dying, Pandora gave Hester a necklace with an eye symbol, and Hester escaped.
Valentine ordered the Guild of Engineers to re-built MEDUSA using stolen components from Tom's museum workshop. His daughter, Katherine, who became estranged from her father, especially after Apprentice Engineer Bevis Pod informed her that Valentine pushed Tom down the chute, learned Valentine's energy project in the re-purposed St Paul's Cathedral was a weapon. Magnus Crome questioned Valentine, and was killed in a coup. Valentine gained support from Londoners by vowing to destroy the Shield Wall with MEDUSA and lead them to a new Hunting Ground in Asia.
Anna Fang took both Hester and Tom Natsworthy to Airhaven in the atmosphere. They met with other Anti-Traction League members onboard. After questioning Hester, Tom realizes Pandora discovered a computer core for MEDUSA. Before they could act, however, Airhaven was attacked by Shrike.
They escaped to Shan Guo. There, Anna convinced Shan Guo Governor Kwan to launch the Anti-Tractionist airship fleet against London, but MEDUSA destroyed the fleet and punched a hole through the Shield Wall. After coming upon a stone carving of Medusa, Hester discovered that her mother's necklace contained a "crash drive", a kill switch for MEDUSA. They scrambled to their airships.
Hester, Tom, Anna, and the remaining Anti-Tractionists began their raid against London. MEDUSA fired a second blast that not only punched a hole into the Wall, but also blew out the Jenny Haniver's windows. The Jenny dived in to avoid the searchlights. The three League airships moved in to attack, but Nils was shot, causing his ship to crash. Yasmina, Anna and Khora continued their attack. Khora's ship was eventually hit and was forced to crash, yet the pilot managed to survive. Yasmina sacrificed herself.
Hester prepped herself with a pistol and dropped from the Jenny. She was then surrounded by guards, but Tom took care of them. Soon, Anna jumped out from her own airship into the roof, with Tom taking over the helm.
Anna met Valentine from the balcony, shooting one of his guards. The Head Historian and she both brandished their swords and engaged in a brutal duel.
While this was happening, Hester entered the chamber and plugged the crash drive into the MEDUSA computer. She punched the code, successfully disabling the weapon. Valentine stabbed Anna, who lived long enough to see MEDUSA shutting down. She escaped.
Exiting the Catheral, Valentine was confronted by Katherine, who begged him to stop his evil ambitions. He shrugged her off, telling her that the Wall was to be fallen.
Valentine's henchmen stormed the control room, killing all personnel and setting London on a collision course with the Wall. Katherine entered to see the carnage. She contacted Tom to open the hatch, allowing the Jenny to fly into and destroy London's engine. He escaped.
Valentine attempted to escape in his airship, only to find Hester before him. She raised her gun at him, yet he took it away. His airship soon shook as Tom destroyed the engines, Hester fought Valentine. She used her knife to injure Valentine, who revealed to her that she was her father. Hester jumped onboard the Jenny, and Tom shot down Valentine's aircraft, sending it to crash. Valentine was then crushed by London's slowing tracks. Hester laughed in victory.
The surviving Londoners, led by Katherine, approached Kwan. They were eventually allowed.