The new adventure finds Will, Chester and Cal reunited on a miners' train plunging towards the Deeps. There they discover the Styx have ambitions to rise up and take over top-soil by releasing a 'Bird-flu'-a deadly strain they have thoroughly tested on the indigenous inhabitants of the Deeps. The boys together with their new found comrades-renegades Drake and Elliott-must find a way to foil the Styx plans and save the top-soilers.