Just as the six mermaid friends think they might make it back to Coral Kingdom, they swim across a mysterious shipwreck. Holly realises that something is wrong, and it's up to her to be very brave if she's going to help her friends. But can she save the home of a family of turtles from danger, too?

Racing to bring the magic crystals to the Coral Kingdom, the six young mermaids discover a dolphin trapped in a fisherman's net, thanks to the evil Mantora, and Sophie, the fastest swimmer, rushes off to find help.

As they continue their quest to find the Snow Diamonds, Katie and her mermaid friends seek help from the Chief Walrus, but first they must save his young grandson, who sneaked away to play with his best friend, a polar bear.

Having had many struggles to bring back all the magic crystals, the mermaids plan to celebrate but the evil Mantora has other plans and it is up to shy, timid Lucy to find a way to save the Coral Kingdom once and for all.