Gregor, his sister, and a group of Regalians go on a journey to rescue Gregor's father and recruit allies for a war against the rats. Though he sympathizes, Gregor is reluctant to help until he learns a surprising fact: his father, who disappeared unexpectedly over two years before, had actually fallen down into Regalia just like Gregor and Boots and been taken prisoner by the rats. The Regalian council believes Gregor to be this warrior, and tries to convince him to undertake the quest mentioned in the 'Prophecy of Gray'. It is then that he learns the real reason for the rats' hatred of Overlanders: a mysterious prophecy written by the human colony's founder Bartholomew of Sandwich hints that an Overland 'warrior' will stop an attempt by the rats to take over the underground realm once and for all. At first, Gregor wants only to return home, but when he is attacked by two rats during an escape attempt and saved by the Underlanders, he inadvertently brings the conflict between the two groups to a head. They meet several 'Underlanders', among them the Regalians' crown princess Luxa, her cousin Henry, and the bats who are 'bonded' to them. The two fall miles below into the Underland: a subterranean world home to humans with near-translucent skin giant sentient bats, rodents, and insects and an escalating conflict between the human city of Regalia and the rats' King Gorger.

When Gregor's baby sister Boots falls through an old air duct grate in the building's basement, he dives in after her. Eleven-year-old Gregor is left home alone in his family's New York City apartment to watch his sister and grandmother.

Gregor underland