In the vibrant underwater kingdom of Pacifica, a magical event occurs when eight mer-persons are chosen by a shimmering beam of light emanating from a colossal golden pearl to become its guardians. However, at the outset, only six of them are present, which prompts Vivian Lu’s character, Isla, to gasp and murmur softly to herself as she swiftly swims away to find the remaining two chosen guardians. In her quest, Isla discovers that these are the "Malibu" Barbie and "Brooklyn" Barbie, both of whom belong to the familiar Barbie universe. Using a special pearl necklace she was given during her appearance in Barbie: Dolphin Magic, Isla helps the Barbie-girls and the "Malibu" Barbie’s sisters—Kirsten Day’s Skipper, Cassandra Lee Morris’s Stacie, and Tatiana Varria’s Chelsea—transform their lives by turning them into mermaids. For Skipper and Stacie, this transformation is a whole new experience as they embrace their first moments of mermaid life, while Chelsea, who had already experienced this in her own series Barbie: Dreamtopia, gets to revisit the magical aquatic world again.
The story reveals that Brooklyn Barbie embarks on her journey of discovery, learning that the legends surrounding mermaids hold true. Meanwhile, at a local science academy, Vivian Lu’s Marlo—a returning character from Dolphin Magic—struggles to convince her colleagues about her scientific proof that mermaids are real. Her attempts are met with skepticism and outright dismissal, as some colleagues doubt her expertise. The academy’s leader, Dr. Amy Tang, dismisses her research as "frivolous fairy-tale witch-hunt" and withdraws funding, further intensifying Marlo's dedication to uncovering the truth about mermaids. Her unwavering focus on her research prompts her to enlist her loyal assistant, Oslo, in her quest to prove that mermaids are not just mythical stories but real beings.
As the narrative progresses, Isla explains her purpose for summoning the girls: they are destined to gain powers and meet Coralia, the revered leader of Pacifica. However, their journey is not without hurdles, as they face off against three initial rivals vying for the coveted title of "Power Keeper"—the formidable Talluyah, Sareena, and Finn. Notably, Finn’s sister, Aquaryah, plays a pivotal role in many of the film’s adventures, particularly teaming up with Chelsea for a series of exciting escapades. During one scene, Marlo captures Chelsea and Aquaryah with fishing nets, using her purple torch-like fish creatures to threaten them. But the resourceful Chelsea quickly uncovers the true motives behind the creatures, allowing the girls to escape their grasp.
Later, "Malibu" Barbie reunites with Marlo, but their peace is disrupted by a submarine crash and unforeseen troubles that force Barbie to seek assistance. Marlo, ever willing to help, steps in to support her. Throughout her journey, Barbie witnesses her fellow mermaids, including her own sisters, demonstrating their unique powers, and finally, she herself gains her water-manipulating abilities. As a team, the mermaids use their newfound powers to tackle an impending environmental disaster—a mountain island overwhelmed with trash and debris threatening both Pacifica and the entire aquatic ecosystem. Aquaryah and Chelsea reunite with Pacifica’s residents, where Chelsea, recognizing Aquaryah’s bravery and kindness, happily declares her the "Power Keeper" for exemplifying the qualities needed to protect their underwater world.
"Malibu" Barbie then leads the group in channeling their powers against the trash island, with Aquaryah initially weak but ultimately empowering herself to nullify the pollution, transforming it into harmless white sprinkles. The film closes on a celebratory note, with the mermaids and their friends commemorating their success in safeguarding Pacifica and its marine life. The final scene captures everyone together, proud of their efforts to preserve their beautiful home, underlining the themes of bravery, friendship, and environmental stewardship.
