In the lagoon near Old Mouse City, cavemice have discovered a giant oyster. It holds a pearl of megalithic proportions! Since the mice of the lagoon don't have the tools to fish the oyster out of the water, they call on Geronimo Stiltonoot to help. Can figure out how to get the pearl before the saber-toothed tigers catch wind of it?

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.