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?

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.

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.

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.

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.