In this gripping fantasy, Col lives on the Upper Decks of the juggernaut Worldshaker, which is a mobile city as big as a mountain. Far below, the 'Filthies' toil amidst the grime, powering the massive engines of Worldshaker. Chosen to be the next Supreme Commander, Col's comfortable life is upset when a 'Filthy', escape and turns up in his cabin, begging for help. Will he hand her over, or will he break all the rules? Col's safe, elite world is about to fall apart.