In his New York apartment, 11-year-old Gregor catches cockroaches and thinks about the giant, sentient cockroaches in the Underland, a fantastical underground world. He lives with his mom; his grandma; his young sisters, Lizzie and Boots; and his ailing father, who has been ill for a few months since Gregor and Boots rescued him from huge rats in the Underland.

Gregor works for his kind neighbor, Mrs. Cormaci, to help earn money for the family. His dad is too sick and traumatized from the Underland torture to return to teaching, and his mom works two jobs. Gregor wants to help, especially since Christmas is approaching. In exchange for doing tasks like cleaning for Mrs. Cormaci, Gregor accepts food and money. He uses the money for things like medicine for his dad and a puzzle book for Lizzie.

Gregor takes two-year-old Boots sledding at Central Park. While he’s lost in thought about helping to provide for his family, Boots disappears.

Panicking, Gregor frantically searches Central Park for his sister. He finds a roach’s leg, sees a large rock blocking a hole in the ground, and finds one of Boots’s mittens nearby. These clues suggest that she was taken underground by the cockroaches, who worship her as a princess.