Ariel is a 16-year-old mermaid princess in the kingdom of Atlantica, in the Atlantic Ocean. She is fascinated by the human world above. She ventures out with her best friend Flounder to collect human artifacts in her grotto and often goes to the surface of the ocean to visit Scuttle, a seagull who offers inaccurate knowledge of human culture. She swims back to Atlantica and is watched by two eels named Flotsam and Jetsam under orders from Ursula the sea witch. Ariel 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 with humans is forbidden.
One night, Ariel, Flounder, and an unwilling Sebastian travel to the surface to watch a celebration for Prince Eric’s birthday on a ship. Ariel instantly falls in love with Eric. Shortly afterward, a violent storm arrives, which wrecks the ship and tosses Eric overboard. Ariel rescues him 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 the girl who saved and sang to him; 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 in her grotto and destroys the artifacts she collected with his trident. After Triton leaves, Ariel begins weeping. Flotsam and Jetsam convince her to visit Ursula.
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 to remain a human permanently. Otherwise, she will transform back into a mermaid and belong to Ursula. Ariel accepts and is given human legs and taken to the surface by Flounder and Sebastian. Unbeknownst to Ariel, Ursula, who harbors a secret vendetta against King Triton, plans to use her as a bargaining chip and take over the kingdom. 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. Angered at Ariel’s near success, 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 a hypnotic enchantment on Eric to make him forget about Ariel and marry Vanessa instead.
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. While Sebastian informs Triton, Scuttle disrupts the wedding with the help of various sea animals. Through all of the commotion the nautilus shell around Ursula’s neck breaks, which restores Ariel’s voice back to her and breaks 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 reveals herself and kidnaps Ariel. Triton confronts Ursula and demands Ariel’s release. However, the deal is inviolable; Triton makes a new deal to take Ariel’s place as Ursula’s servant by giving up his trident in order to save his daughter. 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, Ariel confronts Ursula and attacks her, and Eric intervenes with a harpoon and hurts Ursula. Ursula attempts to kill Eric, but Ariel intervenes and causes Ursula to inadvertently kill Flotsam and Jetsam. As Ariel hurries to join Eric, the 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 gains full control of the entire ocean, creating a storm and bringing sunken ships to the surface. As Ursula is about to destroy Ariel with the trident, Eric steers a wrecked ship and kills Ursula by impaling her with its splintered bowsprit. With Ursula dead, Triton and the other polyps in Ursula’s garden revert to their original forms. Realizing that Ariel loves Eric, Triton changes her from a mermaid into a human permanently and approves her marriage to Eric. At the wedding, Ariel and Eric marry on a ship and depart.