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!

The treasure in question is the Casket of Desires, which contains a sleeping mermaid who has the power to grant any wish. But it is hidden in the deepest darkest part of the ocean, protected by the most hideous, fearsome sea-creatures known to pirate lore.

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. The pirate with the biggest beard-jumper in the nautical world - Captain Black Beard-Jumper - is also on the look out for the Casket and he'll curse anyone who gets in his way...