Col Porpentine understands how society works: The elite families enjoy a comfortable life on the Upper Decks of the great juggernaut Worldshaker, and the Filthies toil Below Decks. Col’s grandfather, the Supreme Commander of Worldshaker, is grooming Col as his successor.

Used to keep Worldshaker moving, Filthies are like animals, unable to understand language or think for themselves. Or so Col believes before he meets Riff, a Filthy girl on the run who is clever and quick. If Riff is telling the truth, then everything Col has been told is a lie. And Col has the power to do something about it—even if it means risking his whole future.