For cavemouse Geronimo Stiltonoot, life is one prehistoric problem after another! 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?