My sister, Thea, had discovered an enormous 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!

Geronimo Stilton is a best-selling children's book series published by Edizioni Piemme of Milan, Italy, since 2000. Scholastic Corporationhas published the English version of the series since February 2004. Although the series credits its title character as the author, the idea was originally thought of by Elisabetta Dami. On older novels, the copyright is issued to a pseudonym "Geronimo Stilton"; this was changed on newer books. The books are intended for readers in the 6–12-year-old age range.