On his way to visit his aunt Sweetfur who is on vacation in the Pleasant Paw Hills, Geronimo Stilton--editor of "The Rodent's Gazette"--gets lost in a dark forest and seeks shelter in a run-down mansion unfortunately haunted by cats.

While searching for her missing baby, a mother mouse encounters a gorilla who chases her all over the world, from China to the Arctic, and when he finally catches her, she gets an unexpected surprise.