Finn's childhood in the valley is idyllic, but across the plains lies a threat. Engn is an ever-growing steam-powered fortress, that needs a never-ending supply of workers. Generation after generation have been taken away, never to return. The Masters of Engn first take Finn's sister, then his best friend, Connor. He thinks he, at least, is safe - until the day the ironclads come to haul him away too.