In 1863, where an
alternate nineteenth century Europe has made tremendous strides in steam-powered technologies, scientist Lloyd Steam and his son, Edward "Eddy" Steam, have succeeded in discovering a pure mineral water in
Iceland which they believe can be harnessed as a nearly unlimited power source for steam engines. An experiment in
Russian Alaska goes terribly wrong, with Eddy being engulfed in freezing gases, but results in the creation of a spherical device. Three years later, Lloyd sends the device, along with its schematics, to his grandson (and Eddy's son), James Ray Steam, who works as a maintenance boy in
Manchester, England, along with instructions to guard it. Members of "The O'Hara Foundation" arrive and attempt to steal the sphere, but Lloyd appears, stating that the device killed Eddy and bids Ray to flee and deliver the device to
Robert Stephenson. After fleeing the O'Hara agents on his personal steam-powered
monowheel, Ray briefly meets Stephenson on a train headed into Manchester, but Ray, along with the device, are captured by O'Hara agents in a
dirigible. Ray is taken to
London, where preparations are being made for the 1866
Great Exhibition, and meets Scarlett O'Hara St Johns, the spoiled granddaughter of the Foundation's head chairman, Charles O'Hara St Johns. He also discovers Eddy, alive but severely scarred and
mechanized from his injuries in Alaska, working with O'Hara on the "Steam Castle"; an elaborate facility entirely powered by three devices called "Steam Balls", one of which was the device sent to Ray. The latter is enamored both by the castle, and his father's vision of using it to enlighten mankind, and volunteers to help complete the project. He also begins developing a
love/hate friendship with Scarlett, who seems to be attracted to him. Ray encounters Lloyd again, who was captured by O'Hara's henchmen, but has escaped from his cell and is attempting to sabotage the castle, revealing that the Foundation's true intentions for the exhibition is to
sell weapons of war. Lloyd shows Ray a hidden armory filled with massive steam-powered war machines built by Eddy, and Ray struggles with the moral dichotomy of being a scientist — of how to contribute to the world without giving into vanity, leaving him conflicted as to whether to side with Lloyd or Edward, who themselves have become estranged. Lloyd and Ray reach the core of the castle and remove one of the Steam Balls, but Lloyd is shot and recaptured by O'Hara, as Ray escapes with the device. The next morning, the exhibition is underway, and Ray brings the ball to Stephenson, giving him the ball and the knowledge he acquired in captivity. The British military attempts to arrest Eddy, but Eddy unleashes his steam powered weapons on the soldiers, turning the exhibition into a battlefield. When Stephenson uses the Steam Ball to enhance his own company's
tanks, Ray realizes that he had no better intentions than the Foundation. Eager to show the world the castle's true power, Eddy orders it to be prematurely activated, causing the structure to rise and shed its decorative outer shell, revealing it to be a monstrous floating fortress. As the
Royal Navy and Stephenson attempt to defend London from the floating fortress, Ray steals the Steam Ball back and uses it to create a makeshift aircraft to re-board the fortress. Ray meets Eddy, Scarlett, and Lloyd in the castle's control room, where Lloyd confronts Eddy about his actions, before shooting him in a fit of rage, but Eddy falls into a cloud of steam and disappears. With the castle steered off course, the structure has become unstable and threatens to explode over the city. As Lloyd and Ray rush to redirect the castle over the
Thames, Eddy, whose metal body repelled the gunshot, re-emerges from the steam and assists them, having realized his folly. Ray re-installs the stolen steam ball, and makes his way to the control room to make a final escape with Scarlett on an emergency jetpack, while Eddy and Lloyd reconcile and halt the machine over the river and escape as well. The castle detonates in a spectacular explosion, sparing the city from destruction. The ending montage reveals Ray returning home, and later becoming a global superhero using the jetpack gear from the castle; Lloyd introducing Ray to electricity and finally passing away; Eddy founding a corporate conglomerate; Scarlett maturing and becoming a famous pilot, and a war being fought with
paratroopers and
zeppelins. ==Characters==