Four months after the Pennine Fusiliers vanished from the Somme, they are still stranded on the alien world. As Lieutenant Everson tries to discover the true intentions of their alien prisoner, he finds he must quell the unrest within his own ranks while helping foment insurrection among the alien Khungarrii. Beyond the trenches, Lance Corporal Atkins and his Black Hand gang are reunited with the ironclad tank, Ivanhoe, and its crew. On the trail of Jeffries, the diabolist they hold responsible for their predicament, they are forced to face the obscene horrors that lie within the massive Croatoan Crater, a place inextricably tied to the history of the alien chatts and native urmen alike. Above it all, Lieutenant Tulliver of the Royal Flying Corp, soars free of the confines of alien gravity, where the true scale of the planet's mystery is revealed. However, to uncover the truth he must join forces with an unsuspected ally.