It's the beginning of summer and everyone is going off to camp except Gregor, an 11-year-old boy from New York City. Since his father's disappearance from their New York City home, he has been helping out with taking care of his little sisters, especially two-year-old Boots. Gregor sacrifices his chance to go to camp, letting his other sister go instead. While doing laundry in the basement of his apartment building, Boots disappears down an air chute and Gregor goes after her. At the bottom of this inner-city rabbit hole, the two find themselves in the Underland surrounded immediately by giant cockroaches, or crawlers. The story moves quickly as the two Overlanders are taken to the Queen of the Underlander humans, where Gregor learns of a prophecy which focuses on him and a quest to find his missing father. He travels with bats, crawlers, spinners (giant spiders), a rat, and two of the royal Underlanders. Gregor spends his time between protecting Boots, who doesn't know she needs to be protected, and becoming the leader of the questors who must not only save his father, but also save the Underlanders' kingdom.