When eleven-year-old Gregor and his two-year-old sister are pulled into a strange underground world, they trigger an epic battle involving men, bats, rats, cockroaches, and spiders while on a quest foretold by ancient prophecy.

Eleven-year-old Gregor and his younger sister, Boots, return to the Underworld beneath New York City to find the cure for a terrible plague that threatens the life of their mother, as well as the lives of the people, bats, and rats who populate the underworld.

When twelve-year-old Gregor finally learns the ancient prophecy, which foretells his death, he must gather his courage to defend Regalia from the army of rats, take his mother and sister home safely, and fight his own dark side.