Spies have reported the sighting of a Rat King in the Underland, a character who has been legendary since the Middle Ages. Recognizable by its tremendous size and snow-white coat, the Rat King is destined to bring war to the Underland.

But when Gregor eventually comes face to face with the Rat King, he finds he is unable to kill him. Gregor's heart tells him he's making the right decision. Will it be one he lives to regret?