My sister, Thea, had discovered an enormouse old castle built by the founder of New Mouse City! She and Trap dragged me along with them to explore it. Inside, we found one hundred keys... but only ninety-nine doors to unlock. What mysterious room did the hundredth key open? It was up to us to find it and reveal its secrets!

BONUS! After the story, read an extra Mini Mystery adventure and jokes galore!

 

This book is part of a book series called Geronimo Stilton .

There are 224 pages in this book. This book was published 2017 by Scholastic US .

In his spare time, Mr. Stilton collects antique cheese rinds. But what he most enjoys is writing books where he tells his fabmouse adventures!