Barbie in A Mermaid Tale follows the story of Merliah Summers, a talented sixteen-year-old surfer from Malibu who is celebrated as the "Queen of the Waves". During an exciting surfing competition on Mavericks, Merliah notices that one of her hairs suddenly turns pink upon contact with seawater, which initially worries her about being perceived as different. As she dives underwater to escape her nerves, she encounters a friendly pink and purple dolphin named Zuma, who reveals that Merliah has the ability to breathe underwater. This discovery sparks her curiosity and leads her to a startling revelation about her true heritage.
Merliah's grandfather, Break, explains that she is half-mermaid, the daughter of a human named Rip and a mermaid queen named Calissa. He tells her that her mother is the reigning Queen of Oceana, a hidden mermaid kingdom. Merliah learns that her mother, Calissa, had her in secret, fearing the evil intentions of her sister, Eris, who seeks to take over Oceana and reign with tyranny. Because Merliah was born without a tail, Calissa took her to stay safe on land, away from her aunt's grasp, entrusting her upbringing to her grandfather.
Initially skeptical and overwhelmed, Merliah shares her findings with her friends, including Hadley. But things take a dramatic turn when Zuma confirms her mermaid origins and reveals that her mother is still alive but imprisoned in Oceana, which is now suffering under Eris's rule. Merliah's discovery is marked by her emotional reaction: she tears off her necklace, which reveals a hidden message about her mother and the fact that her mermaid powers are real. From this moment, her resolve to rescue her mother and restore peace to Oceana is set.
To aid her quest, Merliah teams up with Zuma and a group of helpful mermaids, Kayla and Xylie, who own a boutique and introduce her to her mermaid identity by giving her a tail. They embark on a mission to find three powerful artifacts needed to overthrow Eris: the Celestial Comb, a Dreamfish, and Eris's protective necklace. Their journey takes them to dangerous locations guarded by giant jellyfish and energetic currents, requiring clever teamwork and persistence.
Merliah demonstrates her growing courage and aquatic skills as she successfully obtains each item. During their adventures, she seeks the help of the prophetic Destinies, three stylish mermaids who are also imprisoned by Eris. Their prophecy reveals that Merliah — the daughter of Calissa — is the key to restoring harmony to Oceana.
As Eris's tyranny intensifies, Merliah confronts her, leading to dramatic clashes at the Eris Festival, where the mermaid kingdom's fate hangs in the balance. Using the artifacts, Merliah and her friends trick Eris and successfully retrieve the Celestial Comb and the Dreamfish. In a pivotal moment, Merliah's refusal to wish away her troubles with the Dreamfish empowers her, granting her a genuine mermaid tail — a symbol of her true heritage and strength.
In the climax, Merliah defeats Eris by leading her into a whirlpool, which results in Eris being banished to the deepest ocean trench. Freed from her evil sister, Calissa makes Oceana healthy once more, and Merliah's mother regains her throne. Calissa also bestows upon Merliah a special necklace that allows her to switch between human and mermaid forms at will.
The adventure concludes with Merliah returning to land, winning a surfing trophy, and embracing her identity as both a girl and a mermaid — a hero who has saved her kingdom and discovered her true self. Her journey combines self-acceptance, bravery, and the importance of family, highlighting that sometimes the greatest strength lies within.
