With two prophecies fulfilled, Gregor is now focused on the Prophecy of Blood, which calls for Gregor and Boots to return to the Underland to help ward off a plague. But this time, his mother refuses to let him go...unless she is allowed to travel with them. When they arrive in the subterranean city, the plague is spreading, and it claims one of Gregor's closest companions. Only then does Gregor start to understand how the illness plays with the fate of all warm-blooded creatures. But how can he help combat it?