Gregor swears he will never return to the Underland, that strange world below New York City. But he is a key player in another prophecy, this one about an ominous white rat called the Bane. The Underlanders know there is only one way to lure Gregor back to their world: by kidnapping his little sister, Boots. The Bane is destined to bring a World War to the Underland. But when Gregor comes face to face with the Bane, he finds he is unable to kill this creature. His heart tells him he's making the right decision. Will it be a decision he lives to regret?
Now Gregor's quest reunites him with his bat, Ares, and the rebellious princess Luxa. They descend into the dangerous Waterway in search of the Bane. If Gregor does not fulfill the prophecy, his life, and the Underland, will never be the same.