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.

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.