Barbie stars as Merliah, a surfer who learns a shocking secret: she's a mermaid! She and her dolphin friend set out for an undersea adventure to rescue her mother, the queen of Oceana.
Merliah, the young queen of the quick Malibu waves, thinks that she is an ordinary teenager; however, after an unexpected incident during the surfing tournament, she discovers her true identity: she's a graceful mermaid. Now, with her new best friends--the happy bubblegum-pink dolphin, Zuma, and the beautiful mermaids, Kayla and Xylie--they embark on a fascinating underwater adventure to save the ocean kingdom of Oceana, and Merliah's mother, Calissa. The girls need to find three magical items. Will Merliah, the princess of Oceana, accept the great responsibility?—Nick Riganas
Barbie in A Mermaid Tale is about a sixteen-year-old girl called Merliah Summers, (Liah for short). She is a top surfer at Malibu and is nicknamed, "Queen of the Waves". On the day of a surfing competition, Merliah is surfing mavericks, when one part of her hair suddenly turns pink on contact with seawater. Nervous of showing everyone her hair & being branded a freak, she dives underwater and meets a pink & purple colored, dolphin named Zuma. Merliah finds out she can breathe underwater.
After telling her grandfather what happened, Merliah finds out that she is half-mermaid. Thinking that her parents died when she was a small baby, Merliah's grandfather tells her the truth. Break tells her that Merliah's father was a human named Rip and that her mother was a mermaid named Calissa (who in fact, is the Queen of Oceana). Break tells her that when Merliah's father died, Calissa was worried about the fact that Calissa's evil mermaid sister Eris would try to rule over Oceana. At the same time, Calissa gave birth to Merliah. Merliah had no tail which made it dangerous for her in Oceana. Calissa didn't want Merliah to get hurt, so she took Merliah to her grandfather, Break. Calissa asked if Break could raise Merliah so that she would be safer on land from Eris's rule. At first, Merliah didn't believe Break and left to tell her friends what happened.
After telling her friends the news, Hadley was excited and believed that Merliah is of mermaid royalty. Just then, Zuma appears and says that it is true about what Break said, but Merliah at first didn't believe anything and in anger she throws the necklace she'd been wearing ever since she was an infant, it reveals Calissa is still alive, spinning Merillia, the life force of the sea. The image reveals that Calissa is also a prisoner. She also tells Merliah that the reason her hair is pink, that she can breathe underwater, is because she is part mermaid. Merliah decides to try to rescue her mother and save the kingdom.
With Zuma as her guide, Merliah arrives in Oceana just when the "Eris Festival" starts. Zuma explains that ever since Eris ruled Oceana, her Merillia was not as strong as Calissa's, which made the ocean weak. Most of the coral has turned brown, much like the autumn leaves. During the "Eris Festival", Merliah and Zuma hide in a nearby shop, where Zuma gets Merliah a tail to escape detection from Eris. Merliah is terrified, as Eris controls Oceana in an evil way trapping people she doesn't like in whirlpools & sending them to the deepest depths of the ocean and doubts she can save Oceana.
After the festival ends, Zuma takes Merliah to a boutique which is owned by two mermaids, Kayla and Xylie. They chose to help her after their pet seal Snouts licks her ticklish toes making her laugh, and they make Merliah look like a mermaid. The team seeks out the Destinies, three small stylish mermaids who have prophetic powers. The destinies are also prisoners of Eris & tell her that Calissa's daughter is already in Oceana to get rid of Eris & return the rule of Calissa. Just before they arrive, they hear Eris storming that she will destroy Calissa's daughter. Merliah hides, while Kayla and Xylie point Eris in the wrong direction. While Eris is distracted, Merliah and Zuma meet the Destinies. They reveal in order for the team to succeed in overthrowing Eris, they need "three tools that are spread far and wide": The Celestial Comb, a Dreamfish and Eris' protective necklace.
They set out to look for the items. First, with Fallon and Hadley's via the internet, they find the Celestial Comb in a cave guarded by three giant jellyfish. Kayla, Xylie and Zuma lure the jellyfish away while Merliah and Snouts look for the comb. Merliah manages to get it and escapes from the cave just before it caves in. Next, they find the Dreamfish, yet again with Fallon's and Hadley's internet help, in the Adenato currents. Merliah gets one to promise to grant her a wish later on by impressing it with her surfing skills. Lastly, they get Eris' protective necklace by distracting her with some music and a song.
Furious, Eris spins a whirlpool in order to get rid of Merliah. Merliah gets sucked into the whirlpool. Desperate, Merliah calls on the Dreamfish which offers to send her back to Malibu, her mermaid half to go away and it as if none of this has ever happened. Merliah hesitates and rejects the wish. The Dreamfish instead gives her a real, strong mermaid tail, and successfully escapes the whirlpool. Merliah throws Eris' necklace to the ground, revealing Calissa spinning Merillia. Merliah finds out that though Eris is from the royal family, she can't spin Merillia, which is why Eris did not destroy Calissa. Eris is fed up with Merliah and goes after her. Merliah leads Eris into her own whirlpool and Eris is sent to the bottom of the deepest trench in the ocean.
Merliah sets Calissa free and Calissa makes Oceana healthy once more. Calissa gives Merliah her necklace that will let her wish to be human or mermaid at any time. Merliah then goes back to land and wins the surfing trophy.
