The mice have asked Luxa, princess of Regalia, for help. She, Gregor, and their bats Aeres and Aurora must go back into the jungle to help them.

School’s out for the summer, and Boots and Gregor are free to go to the Underworld to see their mother, and play with their Underworld human and animal friends. But in the middle of a party, a large bat brings a message to Luxa, the princess of Regalia, from a mouse at Queenshead. The mice are in terrible danger. Luxa and Aurora, Gregor and Aeres fly off to Queenshead, only to discover that the mouse who’d sent the message has been killed. Her name was Cevian, and she had once saved their lives. Now the only way to find out why Cevian summoned Luxa is to go into the jungle and meet with the mice.

But when Aeres and Gregor move Cevian’s body to a cave, they discover a strange mark where her body had lain. They are sure she was trying to write something just as she died. Perhaps P for Princess Luxa. Perhaps R for Ripred, the rat who would be king. Or B for Bane—the white rat that Gregor was unable to kill when he was a baby, even though the Prophecy of Bane had foretold it. Bane is eight feet tall now, angry, vengeful, and dangerous.

There’s no way to know what Cevian was trying to say, unless Gregor and Luxa and their bats return to the jungle, meet with the mice, and find out what is making them so desperate that they have to summon a princess to their aid.