Gregor and the Curse of the Warmbloods is the third book in the Underland Chronicles. It was published in the U.S. on July 1, 2005 by Scholastic Press.

After receiving many letters from the Underlanders about Luxa's whereabouts, Gregor finds a note in the laundry room. Hoping to find out where his friend is, he discovers that the Prophecy of Blood has struck, and his bond, Ares, is infected with the plague.

In order to convince Gregor's mother to let him go back to the Underland to find the cure, Ripred sends a message to their apartment. His mother agrees, under one condition: she must accompany her children to the Underland.

When they arrive in Regalia, they are checked to see if they have contracted the plague, and are ushered to a meeting regarding the quest. At the meeting, a bat named Icarus crashes, and the body is burned, sending fleas scattering.

After regrouping, it is discovered that Gregor's mother has been bitten, infecting her with the plague.

In order to save his mother and his friends, Gregor embarks on a treacherous journey through the jungle, uncovering secrets that may shift the uneasy balance of peace in the Underland.