Col is a wealthy child of privilege. Raised to succeed his grandfather as the Supreme Commander of the juggernaut Worldshaker, he has lived a pampered life on the Upper Decks. He has never questioned his place in the world or his bright and illustrious future. But when a Filthy girl stows away in his cabin, suddenly nothing is clear anymore. Quick and clever, Riff is nothing like the Filthies that Col always learned about—the dumb, slow, less-than-human folk who toil away Below, keeping Worldshaker moving. Filthies are supposed to be animal-like, without the power of speech or the ability to think for themselves—but Riff is clever and quick and outspoken, and Col is drawn to her despite himself. As Col begins to secretly spend more time with Riff, he begins to question everything he was raised to believe was true, and realizes that if Riff is right, then everything he was raised to believe is a lie. And Col himself may be the only person in a position to do something about it—even if it means risking his future.

Equality remains elusive in this stunning steampunk sequel to Worldshaker . In the aftermath of the events of Worldshaker, the Filthies control the massive juggernaut, now called Liberator. Many members of the former upper class, called Swanks, have remained behind to help teach them how to operate the juggernaut and to build a new society together. But all is not idyllic aboard Liberator. A saboteur seems determined to drive up anti-Swank sentiment among the more volatile Filthy factions. And the Swanks are finding that their best efforts to work with the Filthies are being tossed aside. Even Col, who thought his relationship with Riff was rock solid, is starting to see their friendship crumbling before him. As tensions run high and coal supplies run low, Liberator is on the verge of a crisis. Can Col and Riff unify their divided people before disaster strikes?