Out on the open sea, Prince Eric (Daniel O'Hara) pursues a mysterious voice despite the protests of his guardian, Grimsby (Joshua Vest). Far beneath the waves, the crab Sebastian (Ashby Nye) has prepared a concert that features King Triton's (Andrew Perrius) youngest child, Ariel (Mollie Childers), but she has snuck up to the surface with her best friend, Flounder (Julia Karns). There, they learn about the human treasure she has found from the seagull Scuttle (Isabella McDonald). Meanwhile, the sea witch Ursula (Megan Moore) and her lackey eels, Flotsam (Kai Amado) and Jetsam (Rachel Luetkemeyer), bemoan their exile and plot to regain power from Triton using Ariel as bait. King Triton scolds Ariel for going to the surface and appoints Sebastian as her chaperone. Ariel retreats to her grotto and dreams of living on land. Upon discovering her collection of human objects, Sebastian extols the virtues of the ocean. Distracted by the shadow of a great ship, Ariel swims to the surface and finds the handsome Prince Eric on board. Suddenly, lightning strikes the ship, sending Prince Eric overboard. Ariel darts into action and brings the unconscious prince to shore. Her song wakes Prince Eric, but she has disappeared into the waves. 

Furious about Ariel's contact with a human, King Triton destroys all her precious objects. Flotsam and Jetsam find Ariel in despair and lure her to the sea witch's lair. Ursula offers to change Ariel into a human in exchange for her voice. Prince Eric must kiss her within three days or Ariel will turn back into a mermaid and become Ursula's slave. On the beach, Prince Eric finds Ariel and brings her home to the palace. Sebastian follows to protect Ariel but finds himself in jeopardy in Chef Louis's kitchen. Ariel's silence prompts Prince Eric to find other ways to communicate, like dancing, but he is still haunted by the mysterious voice. On the second day, Sebastian calls on lagoon animals to serenade the couple, but Flotsam and Jetsam cause a commotion that blocks their kiss.

Grimsby arranges a singing contest on the third day to locate the owner of the mysterious voice and thus a bride for Prince Eric. When Ariel breaks through the crowd and dances, Prince Eric realizes how much he loves her and begins to propose. However, the sun sets and the spell is broken, turning Ariel back into a mermaid and drawing her into the sea. Sebastian rushes to warn King Triton, who agrees to hand over his trident and take Ariel's place as Ursula's slave. When Prince Eric attempts to rescue Ariel, Ursula loses control of her magic shell and Ariel's voice is restored. Trident in hand, Ursula attempts to harness the power of the seas but creates a whirlpool that destroys her and the eels. With peace restored, Ariel returns the trident to her father. King Triton realizes how much Ariel cares for Prince Eric and decides to restore her humanity. He gently deposits her on the shore, and Prince Eric rushes up and proposes. As Ariel answers, Prince Eric realizes that it has been her voice all along.