This third book continues the saga of Gregor's adventures in the amazing subterranean world under New York City. This time the 11-year-old is trying to save victims of an Underland plague, including Ares, his giant bat bond-mate, and Gregor's own mother. The quest for the cure takes the boy and his friends into new sections of the Underland where plants are carnivorous and a giant lizard serves as a guide.