In the fourth volume of the Underland Chronicles, Gregor is drawn ever deeper into a brewing crisis. For generations, rats have run the mice, or nibblers, out of whatever lands they’ve claimed, keeping them constantly on the move. But now the mice are disappearing, and Luxa, who credits them with saving her life, is determined to find out why. When Gregor joins her on a fact-finding mission, he’s relieved that this time, at least, there’s no prophecy on the line. But when the true fate of the mice is revealed, it is something far more sinister than Gregor or Luxa has imagined—and it points the way to the final prophecy he has yet to fulfill.