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

Bart Barnacle, the prehistoric pirate who has been visiting the cavemice, is ready to return to his pirate island home. It's so far away that the Stiltonoots offer to accompany him on the long, treacherous journey over the sea. On the way, they encounter megalithic danger and hungry sea monsters! What an adventure!