When Gregor's baby sister Boots falls through an old air duct grate in the building's basement, he dives in after her.

At first, Gregor wants only to return home, but when he is attacked by two rats during an escape attempt and saved by the Underlanders, he inadvertently brings the conflict between the two groups to a head.

The quest group is challenged to successfully recruit allies for the Regalians, along the way facing battles with giant spiders, scorpions, and rats.

During the questers' attempt to escape, Gregor sacrifices himself to lure the rats' attack force — King Gorger and Henry among them — off the edge of a cliff.

[1] Beware, Underlanders, time hangs by a thread The hunters are hunted, white water runs red.

The phrase "white water runs red" refers specifically to how the humans deposit the bodies of two rats who try to kill Gregor into a fast-flowing river beneath the Regalian palace.

[6] Meaning: Henry, the last "quester" to die, decides that he is a traitor to the humans, and seals his fate by failing to notice as Gregor leads him off a cliff.

[7] To date, editions have appeared in German, French, Spanish, Norwegian, Italian, Polish, Greek, Dutch, Finnish, and Chinese.

Common Sense Media gave it a 5/5 and credited the book's "strong characters, vivid descriptions, flawless pacing, breathless excitement, laughs and scares, and a vision that makes this fantasy very different.

[12] Collins herself has stated that "she spent hours ... plotting strategic alliances that would make military sense" in a way that kept such themes accessible for younger audiences.

[14][15] The reviewer stated, "Collins does a grand job of world-building, with a fine economy of words-no unnecessary details bog down either the setting or the invigorating story.

In her world, a child singing "Patty-Cake" can change the course of history and a stoic rat can mourn the fact that although he is able to read, he cannot write because he has no thumb.

"[9] The same School Library Journal review which praised the novel's "exciting scenes and cliff-hanger chapters" also had this to say: "Gregor is not the most compelling figure at first, but as the story progresses he becomes more interesting, maturing through the challenges he faces.

"[11] Gregor the Overlander was named a Kirkus Reviews Editors' Choice and placed third for the Nutmeg Children's Book Award in 2006 in the intermediate category.