Ariel is a 16-year-old mermaid princess dissatisfied with underwater life in the kingdom of Atlantica, a fantasy kingdom near the coast of Norway in the Atlantic Ocean. She is fascinated by the human world. With her best friend Flounder, Ariel collects human artifacts in her grotto and visits Scuttle, a seagull who offers inaccurate knowledge of human culture. She ignores the warnings of her father King Triton, the ruler of Atlantica, and Sebastian, a crab who serves as Triton's adviser and court composer, that contact between merpeople and humans is forbidden.
One night, Ariel, Flounder, and an unwilling Sebastian travel to the ocean surface to watch a celebration for Prince Eric's birthday on a ship. Ariel falls in love with Eric. A violent storm arrives, wrecking the ship. Ariel rescues Eric and brings him to shore. She sings to him, but leaves just as he regains consciousness to avoid being discovered. Fascinated by the memory of her voice, Eric vows to find and marry the girl who saved his life, and Ariel vows to find a way to join him in his world. Noticing a change in Ariel's behavior, Triton questions Sebastian and learns of her love for Eric. Triton confronts Ariel and destroys her collection of artifacts. After Triton leaves, two eels named Flotsam and Jetsam convince Ariel to visit Ursula the sea witch.
Ursula makes a deal with Ariel to transform her into a human for three days in exchange for Ariel's voice, which Ursula puts in a nautilus shell. Within these three days, Ariel must receive the "kiss of true love" from Eric. If Ariel gets Eric to kiss her, she will remain a human permanently. Otherwise, she will transform back into a mermaid and belong to Ursula. Ariel accepts and is then given human legs and taken to the surface by Flounder and Sebastian. Eric finds Ariel on the beach and takes her to his castle, unaware that she is the one who had rescued him earlier. Ariel spends time with Eric, and at the end of the second day, they almost kiss but are thwarted by Flotsam and Jetsam. Ursula disguises herself as a beautiful young woman named Vanessa and appears onshore singing with Ariel's voice. Eric recognizes the song and, in her disguise, Ursula casts an enchantment on Eric to make him forget about Ariel.
The next day, Ariel discovers that Eric will be married to Vanessa. Scuttle discovers Vanessa's true identity and informs Ariel, who immediately pursues the wedding barge. Sebastian informs Triton, and Scuttle disrupts the wedding. In the chaos, the nautilus shell around Ursula's neck is destroyed, restoring Ariel's voice and breaking Ursula's enchantment over Eric. Realizing that Ariel is the girl who saved his life, Eric rushes to kiss her, but the sun sets and Ariel transforms back into a mermaid. Ursula then kidnaps Ariel. Triton confronts Ursula and demands Ariel's release, but the deal is inviolable. At Ursula's urging, Triton agrees to take Ariel's place as Ursula's prisoner, giving up his trident. Ariel is released as Triton transforms into a polyp and loses his authority over Atlantica. Ursula declares herself the new ruler, but before she can use the trident, Eric intervenes with a harpoon. Ursula attempts to kill Eric, but Ariel intervenes, causing Ursula to inadvertently kill Flotsam and Jetsam. Enraged, Ursula uses the trident to grow to a monstrous size.
Ariel and Eric reunite on the surface just before Ursula grows past and towers over them. She then creates a storm and brings sunken ships to the surface. Just as Ursula is about to kill Ariel, Eric steers a wrecked ship towards Ursula, impaling her with its splintered bowsprit, killing her. With Ursula dead, Triton and the other polyps in Ursula's garden revert to their original forms. Realizing that Ariel truly loves Eric, Triton willingly changes her from a mermaid into a human permanently and approves their marriage. Ariel and Eric marry on a ship and depart.
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