Col lives in luxury on the Upper Decks of the juggernaut Worldshaker, a mobile city as big as a mountain. Far Below, the Filthies toil amidst the grime, powering the massive engines. Col has been chosen as the next Supreme Commander - but then a girl Filthy escaped from Below appears in his cabin. 'Don't let 'em take me!' she begs. Will Col hand her over, or will he break all the rules? Either way, his safe, elite world is about to fall apart.