Gregor swears that he will never set foot in the Underground, a strange world beneath New York. But the boy does not know that he is a key figure in the next prophecy, which bears the ominous name “The Prophecy of Mortos.” The inhabitants of the Underdark understand that in order for Gregor to come down to them again, there is only one way to lure him - to kidnap his sister Barefoot. And so Barefoot, Gregor, his bat Ares and the rebellious princess Luxa embark on a dangerous journey along the Waterway in search of the huge white rat from the prophecy. If they fail in their mission, their fates and the fate of the Underdark will change forever. “The Underground Labyrinth” is the second book and the second prophecy about Gregor.