Many years ago, a man by the name Bartholomew of Sandwich took a group of 800 humans on a journey deep under the earth's crust. They ended up in the Underland, where they settled into their new lives. The humans with Sandwich built a gorgeous stone city known as Regalia, where the majority of the Underlanders reside. They live in an age of relentless war with the various other creatures in the Underland. Giant six-foot-tall rats known as gnawers are their most common enemies, but the enormous bats (aka fliers) are close allies with them. The giant cockroaches (aka crawlers) usually stay out of the conflicts, and the giant spiders (aka spinners) always help out whichever side is winning.

In the present-day, eleven-year-old NYC resident Gregor discovers an entrance to the Underland, and he literally falls into the hidden world below. He discovers that Sandwich left many prophecies carved into walls, and many of them mention him! His journey becomes incredibly lethal as he fulfills more and more prophecies, and he is left wondering when the prophecies will release their hold over him and let him choose his own fate for once.