Gregor learns of his role in another Underland prophecy, yet he swears he will never return to that place. But his vow crumbles when his little sister Boots is snatched. Gregor knows it is a ploy to lure him to the subterranean world, but he gives in and heads back underground, where he is reunited with his bat Ares, the princess Luxa and new allies. Together they descend into the deadly Waterway in search of an ominous rat known as Bane. But Gregor must face the possibility of his greatest loss yet and make life and death choices that will determine the future of the Underland.