Set a few months after the first book, Gregor must travel to The Labyrinth to kill The Bane and prevent his little sister, Boots, from being killed by the rats

Blurb reads:
Months have passed since Gregor
first fell into the strange Underland beneath New
York City, and he swears he will never go back.
But he is destined to be a key player in another prophecy,
this one about an ominous white rat called the Bane. The
Underlanders know there is only one way to lure Gregor back
to their world: by kidnapping his little sister, Boots.
Gregor's quest reunites him with his bat, Ares and the
rebellious princess, Luxa. They desend into the dangerous
Waterway in search of the Bane. If Gregor does not fulfill the
prophecy, his life, and the Underland, will never be the same.