But when an evil mermaid schemes to take over the ocean kingdom, she finds herself racing the clock to save the merfolk and stop Eris's ambition later finish the big surfing competition.

When the evil mermaid Eris escapes from her whirlpool with plans to take over the throne of Oceana, Merliah and her sea friends dive in to stop her.

Merliah wins first place, narrowly beating rival surfer, Kylie Morgan, though both get to compete in the finals in Australia.

Merliah uses her magical necklace to transform into a mermaid and tell her mother Queen Calissa the good news.

Calissa is preparing for the Changing of Tides Ceremony to be held at the ancient city of Aquellia deep in the waters of New Zealand, which is performed every twenty years to make the next princess of Oceana an official member of the royal family and imbuing them with the power to spin Merillia.

That night, at a luau, Kylie is approached by a rainbowfish named Alistair who tells her that Merliah has powers that give her an advantage with surfing.

While Calissa and the ambassadors of the ocean—Mirabella, Kattrin, Selena, and Renata—are waiting for the midday sun for the ceremony to begin, they are attacked by Eris.

Eris manages to cast her spell on Calissa, whose tail becomes heavy and drags her down, trapping her at the bottom of the ocean.

As Calissa is unable to move because of the spell, Merliah decides to perform the ceremony in her place, even though doing so will make her a mermaid permanently.