The Underland is inhabited by humans who traveled below hundreds of years ago, along with various giant versions of creatures like bats, cockroaches, and rats.

Gregor and his two-year-old sister Boots fall through a vent in their apartment basement and find themselves in a strange place called the Underland, inhabited by giant animals and pale humans.

The Underlanders name Gregor "the warrior" and Boots the "princess" of a prophecy written hundreds of years ago by their founder, Bartholomew of Sandwich.

[2] Gregor is caught up in one of Bartholomew of Sandwich's prophecies once again when Underlanders kidnap his baby sister for her protection, and he reluctantly returns to their subterranean world.

[3] A mysterious plague has broken out in the Underland, and yet another of Sandwich's prophecies leads the warm-blooded creatures of the realm to organize a quest for the cure, supposedly located in a dangerous underground for his friends and more than willing to help, especially as his own mother and bond lie dying in the Regalian hospital.

He soon finds his entire family drawn into Sandwich's deadly Prophecy of Time, and events around him begin to climax towards a final confrontation with The Bane.

[2] Sandwich was a self-styled prophet who believed that a great tragedy would one day befall all those living on the surface, and convinced his devoted followers to establish an underground community long before the event could happen.

This behavior is exemplified by a story which Gregor learns of in the final book, detailing how the humans' first act in the Underland was to poison an entire species living on the land that would become Regalia.

[6] Gregor (and his little sister Boots) spends time with many different creatures over the course of the series, and comes to believe that the humans' xenophobia and unwavering trust in Sandwich's judgment are both wrong and dangerous to everyone in the Underland.

Underland humans are a necessarily warlike race, but their fighting methods are most similar to those of the Medieval Ages due to an inability to mass-produce explosive material, like gunpowder.