Maisie, a clever little beagle, decides to become a scuba diver and explore the ocean.
Mr and Mrs Harris, her unsuspecting owners, believe Maisie is playing safely on the beach. But Maisie is having fun scuba diving in the ocean.
Soon she meets Princess Zara and her mermaid sisters. They have been locked out of their golden palace. The terrifying Octo has stolen the golden key and hidden it in his Cave of Darkness.
Maisie must enter the Cave of Darkness to find the golden key.
Will she be able to help the princess mermaids or will her underwater adventure end in disaster?