The water levels are rising! Becky and her friends have to be quick if they are going to save Ice Kingdom from melting for ever. Becky comes up with a magical plan to help her fellow Sisters of the Sea find the precious Snow Diamond and befriend a young Human girl.

In their quest for the third stolen Snow Diamond, Megan and her mermaid friends must travel fast to the warm Golden Shore. Will they succeed in unravelling the next cryptic clue, as well as escaping Mantora's evil Storm Gulls on their most difficutl mission so far?

Holly must solve the puzzle of an old legend if she and the other mermaids are to stop the toxic waste leaking from a wrecked ship, threatening a family of turtles living nearby, and get back to their vital mission for Queen Neptuna.

Misty and five fellow mermaids, on a quest to retrieve six magic crystals that will protect the Coral Kingdom for the next year, face a storm sent by the evil Mantora and are driven off-course to the Kelp Forest, where they find a different kind of trouble.