Geronimo Stilton and the Hunt for the Hundredth Key

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!