Atticus is on beach-tidying duty with the kittens when he finds a message in a bottle from Fishhook Frank. Fishhook is marooned on a desert island but he promises Atticus treasure in exchange for his rescue! Atticus sets out to rescue Fishhook Frank and find the Casket of Desires with his owners the Cheddar family. But it's not all plain sailing and Atticus must learn to be a first-class ship's cat if they want to stay afloat!