Gregor follows his sister (Boots) down the grate in the laundry room of his apartment building and falls miles down to a place called The Underland where he bumps into giant cockroaches called Crawlers who take them to the underground human city of Regalia, where he meats Queen Luxa and Vikus. He tries to escape whilst being held prisoner in Regalia when he meets two ginormous rats called Gnawers who try to eat him, thankfully, Mareth, Perdita, Henry and Luxa save his life and bring him back to Regalia where he learns that he is a Warrior metioned in a prophecy that goes as follows:
Beware, Underlanders, time hangs by a thread.
The hunters are hunted, white water runs red.
The Gnawers will strike to extinguish the rest.
The hope of the hopeless resides in a quest.
An Overland warrior, a son of the sun,
May bring us back light, he may bring us back none.
But gather your neighbors and follow his call
Or rats will most surely devour us all.
Two over, two under, of royal descent,
Two Flyers, two Crawlers, two Spinners assent.
One Gnawer beside and one lost up ahead.
And eight will be left when we count up the dead.
The last who will die must decide where he stands.
The fate of the eight is contained in his hands.
So bid him take care, bid him look where he leaps,
As life may be death and death life again reaps.
Eventually, Gregor is convinced to help with this quest and as soon as the rats start attacking they are on their way to the Fliers, to get two Fliers to join their quest. After they have two Fliers they go to the Crawlers land to get two Crawlers, but oddly the Crawlers start worshipping Gregor's sister, Boots and agree to join the quest. They then go to the Spinner's land, unfortunatly, the Spinners deny to join the quest and inmprison the adventurers who swiftly manage to escape. After they escape the Spinner's lair, Vikus reveals to the group that him, Mareth, Solovet and their Fliers must leave as the Prophecy only lists two Underlanders, two Fliers, two Crawlers, two Spinners, two Overlanders and one lost up ahead, they also leave them with a new guide called Ripred a rat who is on the adventures side, it is also revealed that Vikus is Luxa's grandfather. The adventurers head into Gnawer territory to find Gregor's father and Henry reveals that he is a traitor and a small army of rats attack them, Gregor realises that the last stanza means that Gregor must jump off the edge of a cliff, Gregor jumps off the edge with all the rats following him, they all fall and surprisingly Ares saves him rather than saving Henry, his bond (bond means they will always save each others lives)