Unlike her little brother, Luke, Rosie doesn't believe that mermaids actually exist. But that's before she finds the stone statue of a mermaid in the middle of the fountain in the grounds of her Great-aunt Rosamund's Scottish castle.
When Rosie curtseys and says hello she finds herself on a deserted beach at nighttime. She can hear beautiful singing coming from some rocks. It's Princess Marissa, the daughter of the king and queen of the mer-people. Rosie finds she can swim underwater and she explores the kingdom of Acquatica with her new friend, Carla and Carlos the dolphins and Octavius the Octopus.
Then Marissa discovers that she's lost her crown, which contains the Pearl of Wisdom and she and Rosie must go and get it back from the old and frightening Sea Hag . . .!