The Hunt for the Hundredth Key is the 100th book in the original Italian Geronimo Stilton series, released in Italy on October 14, 2014, and the english version was released on April 1, 2017.
Plot[]
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!.