Hans Christian Andersen's The Little Mermaid opens in live-action Denmark, where a narrator nods to the author’s origins and his classic tale before the scene gracefully shifts into a rich, 2-D hand-drawn anime world. In the undersea kingdom, Marina, Fumie Kashiyama lives with her father, grandmother, and five older sisters, sharing games with her best friend Fritz the Dolphin, Mariko Miyagi. The sisters’ light-hearted routine is interrupted by a tempest conjured by the Sea Witch, a danger that arrives just as Marina and Fritz enjoy a sunny moment near a sunken ship.
When the storm subsides, Marina’s sisters scold her for being late and remind her that adulthood comes with responsibility—their grandmother will only grant the pearl hairpin that marks maturity if she proves herself. The next day, the sisters venture to the surface while Marina remains below, awaiting her own rite of passage. During a dive among ruins, Marina discovers a statue of a human prince. She and Fritz summon the courage to reach the surface, where she glimpses the prince aboard a ship, sparking a longing that will drive the rest of the tale.
A second storm hurls the prince into the sea, and Marina acts quickly to save him, guiding him to shore and leaving him there to be found. A raven-haired girl on the shore tends to the prince, and Marina’s selfless act earns her grandmother’s blessing to come of age—she receives the coveted pearl hairpin. Yet the wish for closer contact with the human world remains strong, and Marina seeks out the Sea Witch for a perilous bargain: human legs in exchange for her voice, forever relinquishing her mermaid song and the chance of ever returning to the sea as a mermaid.
The Sea Witch makes the swap, warning that if the prince marries another, Marina will die at sunrise of their wedding day. After a heartfelt farewell to her family and Fritz, Marina drinks the potion and undergoes the transformation. She is found by the prince on the shore and, for a month, she and the prince grow closer as he learns to trust and care for her. He speaks of his parents’ wish for him to wed a foreign princess, but he longs to marry the girl who saved his life—yet he cannot locate that mysterious savior.
Trouble intensifies when the prince’s cat, Jemmy the Cat, Kazuko Sawada, grows suspicious of Marina and reports their exchanges to the royal court. The queen—sensing a charm at work—begins to fear Marina’s influence over her son. When the foreign princess arrives, the king seals Marina’s arrest for treason. The prince recognizes the foreign princess as the raven-haired girl who saved him earlier, and the couple’s betrothal seems sealed.
Heartbroken, Marina asks Fritz for one last farewell, and the dolphin promises to help. Marina’s five sisters, who have already traded their own hair to the Sea Witch in exchange for a magic dagger, provide Marina with the weapon and explain a brutal path: if she stabs the prince and lets his blood touch her feet, she will revert to a mermaid. With dawn only moments away, Marina slips into the prince’s room but cannot carry out the act. She kisses him gently and retreats, dropping the dagger as a symbol of what she cannot do.
The dawn arrives, and Marina leaps from the ship into the waiting morning light. The prince and the others rush to call her name, but she sinks into the sea and dissolves into foam, leaving behind a pearl hairpin and a single scale from her tail as proof of her passing. The sun climbs, and the couple—though shaken—watches Marina’s fate with heavy hearts, mourning her loss as the film shifts back to Denmark.
As the live-action frames reappear, Marina awakens in the realm of the Daughters of the Air, discovering she has become one of them. She smiles softly at the memory of the distant prince, the longing for a true connection, and Fritz’s steadfast friendship, then takes to the skies to join her new sisters. They tell her that kind deeds toward people in the world will earn her an immortal soul and, someday, guide her to a true prince. The story closes on a tranquil still shot of the Little Mermaid statue in Copenhagen, signaling both an ending and a hopeful new beginning for Marina.
