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?