It`s only a few months since Gregor and Boots returned from the Underland, leaving their mother behind to heal from the plague. Though Gregor`s family receives frequent updates on her condition, they all know Gregor must return to fulfill his role as the warrior who is key to the Underlanders` survival. Accompanied by his now-talkative little sister Boots, still considered the honorary "princess," Gregor joins forces with another princess--12-year-old Luxa--and Ripred the rat to defend the Underlanders and the vulnerable "Nibblers," or mice, from the rat army.