The mermaids go on a quest to find the stolen snow diamonds and return them to Princess Arctica before the kingdom melts, and Amber helps a young Inuit girl save a beluga whale while Katie Leads the group to find a missing baby polar bear and walrus.
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.
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.
Katie and her friends find the first snow diamond and proceed to solve more of Mantora's riddles, and when a baby walrus and baby polar bear go missing, they attempt to locate them as well as the second diamond.
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.
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.
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.
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.
