Merliah Summers (Liah for short) is an accomplished 16-year-old surfer in Malibu called the “Queen of the Waves”. At a surfing competition, Liah is surfing mavericks. suddenly a portion of her hair turns pink. Embarassed, she dives underwater to hide the change from her friends and not only meets Zuma an odd pinkish/purple dolphin, but also discovers she is capable of breathing underwater.
Liah discovers after discussing this with her grandfather, Break that she is, is half-mermaid. Her father Rip fell in love with the mermaid Calissa. When Rip died, Calissa, the Queen of Oceana became concerned that her evil sister Eris would take the opportunity to depose her. And when she was born, Liah didn’t have a tail, so Calissa decided it would be best if Break raised her on land, since she’d otherwise be threatened by the new Queen. Liah doesn’t’ believe any of this and goes out to talk it over with her friends.
As it turns out, her friend Hadley is thrilled at the news and is completely convinced Liah is a mermaid princess. Zuma shows up and tells her that Break was telling the truth. This is too much for Liah and in anger she takes off the necklace she’s had since a baby into the water. This act puts her face to face with Merillia; the life force of the sea who convinces Liah that she is the mermaid princess her grandfather and Zuma says she is and that her mother is still alive. Merliah finally accepts her nature and make the decision to save her mother and her kingdom.
Zuma guides her to Oceana, and they get there just at the beginning of the “Eris Festival”. The coral has all turned brown, like autumn leaves. Zuma tells Liah that this is because Eris’ Merillia is much weaker than her mothers, and therefore the ocean itself has lost its vitality. They hide in a shop, where Liah witnesses Eris’ power and evil in creating whirlpools to trap her enemies. Scared of such power, she becomes to doubt she can possibly save Oceana. Once the festival has ended, Zuma guides Liah to a boutique owned by the duo of friendly mermaids, Kayla and Xylie. After Snout, their pet seal licks her ticklish toes and making her laugh, they decide to help Liah. They help her appear like she’s a mermaid, then together they all head off to locate the Destinies, a trio of stylish if diminutive mermaids who are said to have the powers of prophecy. Only, once they get there they see that Eris as arrived ahead of them and is angrily vowing to destroy her sister’s daughter. After they make sure Liah is hidden, Kayla and Xylie trick Eris into leaving, Meanwhile, Merliah and Zuma sneak in and talk to the Destinies. The trio of mermaids tells them that if they are to depose Eris and her evil, they must find the “three tools that are spread far and wide”: These three are: the Celestial Comb, a Dreamfish and Eris’ protective necklace.
With Fallon and Hadley’s help, they locate the Celestial Comb in a cavern where it is protected by three colossal jellyfish. Kayla, Xylie and Zuma keep the jellyfish at bay and Liah and Snouts search for the comb. Liah gets out of the cave just before it collapses on her, and triumphantly, with the comb. Fallon and Hadley search the internet and come up with the location of the Dreamfish, it is somewhere in the Adenato currents. Impressing it with her surfing skills, Liah gets the Dreamfish to grant her a wish. Finally, the get their hands on Eris’ protective necklace through tricking her with a distracting song. The enraged Eris creates a whirlpool which sucks Liah in. Liah calls on the Dreamfish to help her, and she is given the option to go back to Malibu to life a human life as though none of her mermaid experiences ever happened, but Liah won’t accept the offer. In response, the Dreamfish imbues her with a strong and real mermaid tail, with which she get out of the whirlpool. Liah throws Eris’ necklace onto the ground, and it shows Calissa spinning Merillia. She learns why her mother is still alive; Eris needs her to spin Merillia. Having had enough, Eris tries to destroy Liah, but ends up being trapped in her own whirlpool and exiled to the bottom of the deepest trench in the ocean. Liah frees her mother and Calissa restores Oceana’s vitality. She then gifts her daughter with a necklace that has the power to maker human or mermaid whenever she wished. Liah then returns to Malibu and goes on to win the surfing competition.
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