The six young mermaids are delayed on their vital mission to bring magic crystals back to the Coral Kingdom by an oil spill created by the evil Mantora, as Ellie seeks the special ingredient for a potion that will help save birds caught in the oil.
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.
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.
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.
The wicked Mantora sets another trap for the young mermaids by littering a beach where a seal family lives, and Scarlett's bossiness interferes with the clean-up effort, further delaying the mission to bring the magic crystals home.
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.
Amber leads her fellow mermaids in trying to help beluga whales that are trapped in ice, while working on solving a riddle from evil Mantora, whose theft of the Snow Diamonds may cause the Ice Kingdom to melt.
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.