Merliah Summers Kelly Sheridan is a young, up-and-coming surfing star in Malibu. During a competition, her focus wavers when her hair suddenly sprouts pink streaks, sending her into a wipeout. While she’s underwater, she discovers she can breathe, and a talking, pink bottlenose dolphin named Zuma greets her. Shocked, Merliah Summers Kelly Sheridan explains the strange encounter to her grandfather, Break Garry Chalk. Break reveals a family secret: Merliah’s mother is a mermaid, and Merliah was given to Break as a baby because she was born with legs. The revelation leaves Merliah overwhelmed and doubtful about her path.
After she confides in her friends, Fallon Nakia Burrise and Hadley Maryke Hendrikse about the tale, Zuma reappears and confirms that Merliah is indeed half-mermaid. Zuma explains that Merliah’s mother is Calissa Nicole Oliver, the former queen of the underwater realm of Oceana. The current ruler, Eris Kathleen Barr, Calissa’s sister, seized the throne years earlier when Calissa vanished. Zuma hopes Merliah will embrace her birthright and challenge Eris. Merliah recoils from the idea; in anger she abandons the necklace she has worn since infancy. When the pendant shatters, a magical image of Calissa appears, proving she is alive. With newfound resolve, Merliah agrees to travel to Oceana to seek Calissa’s aid in reclaiming her life.
On the voyage to Oceana, Zuma explains that the ocean’s vitality is waning because Eris lacks the skill to spin Merillia—the magical substance that sustains the seas. The trio slips into Oceana during a festival orchestrated by Eris, during which she distributes Merillia to win the citizens’ adoration. In truth, Calissa is the one spinning Merillia and is imprisoned in the palace dungeon.
With the help of mermaids Kayla and Xylie, Merliah’s legs are masked by a pretend tail. They seek the Destinies, three mermaids with prophetic powers, who reveal that Merliah must collect three items to overthrow Eris: the Celestial Comb, a Dreamfish, and Eris’s protective necklace. The group pursues the items, occasionally turning to Fallon Nakia Burrise and Hadley Maryke Hendrikse for research when needed. The Celestial Comb is located in an underwater cave, and the Dreamfish lies within the Adenato currents.
To seize Eris’s necklace, Merliah, Kayla, and Xylie confront Eris during the next festival and perform a song and dance to distract her. Merliah manages to grab the necklace, but Eris retaliates by yanking off Merliah’s tail, exposing her legs. Enraged, Eris conjures a whirlpool to banish Merliah to the ocean’s deepest trench. Trapped, Merliah calls on the Dreamfish for help. The Dreamfish offers to grant Merliah’s deepest wish and return her to Malibu, erasing her mermaid half. Merliah declines the offer and embraces her responsibilities as Oceana’s princess, earning a real mermaid tail that helps her escape the whirlpool.
Back above the waves, Merliah reveals her true identity to the Ocean people, brandishing the Celestial Comb as proof. Eris pursues, but Merliah outsmarts her and drags the tyrant into the whirlpool, trapping her. Merliah then locates Calissa and frees her, allowing Calissa to reclaim the throne. With Calissa back in power, she regains the ability to spin Merillia at full strength, restoring Oceana to its former vitality. Merliah confesses that she misses life on land, and Calissa presents her with a magical necklace that lets her transform between human and mermaid at will. The tale concludes with Merliah returning to shore, reuniting with Break Garry Chalk, and entering the surfing competition with renewed confidence.
