Gregor turns around and Boots is gone, taken back to the Underland. He has no choice but to follow and rescue her.

Things have been rough for Gregor and his family since he and his little sister Boots brought their father back from the Underland, a world hidden deep beneath New York City. His father's sick from his years of imprisonment, unable to work, and they are barely scraping by. Then just before Christmas, the giant cockroaches from the Underworld kidnap Boots while she's playing in the snow in Central Park. Gregor is forced to do what he swore he would never do — go back to the Underworld to rescue her. But what he doesn't find out until he's reached the Underworld is that the cockroaches aren't going to hurt Boots. They're taking her to Vikus in the city of Regalia, where she'll be safe. It's the rats of the Underworld that have sworn to kill her to fulfill the Prophecy of Bane.

If Under fell, if Over leaped.
If life was death, if death life reaped,
Something rises from the gloom
To make the Underland a tomb.

Hear it scratching down below,
Rat of long-forgotten snow,
Evil cloaked in coat of white
Will the warrior drain your light?

Who would turn the warrior weak?
What would burning gnawers seek?
Just a barely speaking pup
Who holds the lands of Under up.

Die the baby, die his heart,
Die his most essential part.
Die the peace that rules the hour.
Gnawers have their key to power.

Gregor is the Overlander fated to fight the giant white rat Bane, full of evil and determined to take over the Underland. Boots is his heart, his Achilles heel. If she dies, he will be broken and defeated, and the rats will rule the Underworld. Vikus and the others who rule Regalia ordered the cockroaches to kidnap Boots, knowing she was no longer safe in the Overland, and knowing that Gregor would be forced to follow. Once he is in Regalia, they reveal the truth and convince Gregor that he must once again begin a quest to save the Underworld. Gregor has no choice. Boots is the most important person in the world to him, and as long as Bane lives, she is in danger, in the Overland, in the Underland. So once more, he prepares for battle.