Geronimo Stilton, the top reporter from the Rodent's Gazette, has been delivering the news to the citizens of Mouse City for years. But lately, Sally Ratmousen from the Daily Rat has found a way to steal all the exclusives from Geronimo, presenting them as her own! Geronimo is sure there is a spy in the Rodent's Gazette and sets a trap to catch them! However, the real test is whether Geronimo can keep the identity of the surprise guest a secret from everyone, especially Sally, until the day of Mouse City's hundred-year celebration!