Ariel is a mermaid princess and the youngest daughter of King Triton, ruler of Atlantica. To Triton's displeasure, Ariel harbors a deep curiosity about the human world and, alongside her friend Flounder, regularly visits the seagull Scuttle at the surface of the ocean and collects human artifacts, which she stores in a secret grotto. One night, Ariel rescues human prince Eric during a violent storm and becomes infatuated with him.
Sebastian, a crab who serves as Triton's adviser, unwillingly reveals Ariel's love for Eric to him, leading an enraged Triton to destroy Ariel's grotto, leaving her devastated. Immediately afterwards, Ariel is visited by two eels named Flotsam and Jetsam, who convince Ariel to seek help from Ursula, a banished sea witch. Ursula offers to transform Ariel into a human for three days in exchange for her voice, during which she must receive the kiss of true love from Eric in order for the transformation to be permanent; if she fails, she will turn back into a mermaid and become Ursula's prisoner. Ariel agrees and Ursula takes her voice, storing it in a nautilus shell, and transforms her into a human. Eric soon finds Ariel and takes her to his castle, unaware she is the girl who rescued him.
Ariel is given a tour of the kingdom by Eric and the two bond. At the end of the second day, the two nearly kiss, but are disrupted by Flotsam and Jetsam. Ursula, fearing that Ariel may succeed in kissing Eric, transforms into a human woman named Vanessa and hypnotizes Eric with her singing voice. The two are set to be married, leaving Ariel devastated. However, Scuttle reveals Ursula's deception to Ariel, who rushes to stop the wedding. Scuttle manages to destroy the nautilus shell, restoring Ariel's voice and breaking Ursula's enchantment over Eric, who realizes Ariel is the girl who saved him. Before they can kiss, however, the sun sets and Ariel turns back into a mermaid.
Taking Ariel prisoner, Ursula is confronted by Triton, who cannot break their contract. Ursula offers to release Ariel in exchange for Triton taking her place, which he accepts. Ursula transforms Triton into a polyp and takes his trident and crown, but Ariel and Eric attempt to subdue her. In the ensuing confrontation, Flotsam and Jetsam are killed, and an enraged Ursula expands into monstrous proportions. She creates a whirlpool and traps Ariel at the bottom of it, but before she can kill her, Ursula herself is impaled by Eric piloting a sunken ship brought to the surface by the whirlpool. With Ursula dead, her previous victims are restored to their original forms, including Triton.
Triton, acknowledging Ariel's love for Eric, transforms her back into a human, and she and Eric reunite on the shore. Soon afterwards, the two are married, and the mer-people are allowed to visit the surface and interact with the human world.
- Jodi Benson as Princess Ariel.
- Christopher Daniel Barnes as Prince Eric.
- Pat Carroll as Ursula.
- Kenneth Mars as King Triton.
- Samuel E. Wright as Sebastian.
- Jason Marin as Flounder.
- Paddie Edwards as Flotsam and Jetsam.
- Buddy Hackett as Scuttle.
- Ben Wright as Grimsby.
- Edie McClurg as Carlotta.
- Rene Auberjonois as Louis.