Gregor and the Marks of Secret is an epic fantasy novel, the fourth book in the critically acclaimed The Underland Chronicles by Suzanne Collins.

The group finds the colony mysteriously deserted, though they do discover a "mark of secret" which Hazard says warns of death and sorrow.

During their journey back to Regalia, they learn the Bane has been systematically executing nibblers, which leads Luxa to declare war against all gnawers.

The team eventually locates the missing nibblers, trapped by rats in a natural gas chamber, and is shocked to discover an Underland children's song describes the scene perfectly (and is thus yet another of Bartholomew of Sandwich's dreadful prophecies).

Once there, they part ways: Gregor, Ares, and Temp going to escort the young and injured back to Regalia and spread the news of the rats' plans; the rest returning to protect and mobilize any remaining nibblers.

Gregor wants to join the latter group, but Ripred convinces him to go learn about the "Prophecy of Time" and finally claim Sandwich's sword.

The song's "true" meaning is unknown until the mass death of a group of nibblers, when Boots's dancing mimics the terrifying scene perfectly.

Father, mother, sister, brother, Off they go I do not know If we will see another Meaning: This section is repeated at the end of every verse, and refers to the fact that entire families of nibblers are being executed by the gnawers.

Luxa explains to Gregor that, in times of war, the Regalians attack their enemies with swords and burning oil poured from high places.