Out on the open sea, PRINCE ERIC pursues a mysterious voice despite the protests of his valet, GRIMSBY. Meanwhile, far beneath the waves, the crab SEBASTIAN has prepared a concert for KING TRITON that will debut his youngest child. However, ARIEL has gone with her best friend, FLOUNDER, to the surface, where they learn about treasures of the world above from the seagull SCUTTLE. Deep within her lair, the sea witch URSULA and her lackey eels, FLOTSAM and JETSAM, bemoan their exile and plot to regain power from King Triton using Ariel as bait.
At court, 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 ocean life . In the midst of this revelry, Ariel pursues a passing ship until she encounters Prince Eric. Lightning strikes the ship, sending him overboard. Ariel saves Prince Eric from drowning, brings him safely to shore, then disappears into the waves. Back underwater, the MERSISTERS grill Flounder about Ariel’s odd behavior. When King Triton learns of Ariel’s contact with a human, he destroys her grotto. 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. However, Prince Eric must kiss Ariel within three days or she will turn back into a mermaid and become Ursula’s slave forever. Overcome by her dreams of the world above, Ariel makes the bargain.
On the beach the next morning, Prince Eric finds the mute Ariel and brings her home to the palace. Sebastian follows to protect Ariel but lands himself in jeopardy in the kitchen of CHEF LOUIS. Ariel’s silence prompts Prince Eric to find other ways to communicate, like dancing, but he is still determined to find the mysterious voice. On the second day, Sebastian and the LAGOON ANIMALS 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 for Prince Eric, he realizes how much he loves her and begins to propose. However, Ursula barges in as 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 restores her human form. He delivers her to the world above, where Prince Eric rushes up and proposes. As Ariel answers, Prince Eric realizes that it has been her voice all along.