Prince Eric, eighteen years old, and his manservant Grimsby are aboard a sailing ship. From the sailors, Eric learns about Triton, the ruler of the sea who can control the oceans. He rules a kingdom of merpeople in the watery depths.
Underwater, a sixteen-year-old mermaid princess named Ariel is dissatisfied with life under the sea and curious about the human world. Ariel, with her best friend, a fish named Flounder, collects human artifacts, and visits the surface of the ocean to visit a seagull named Scuttle, who offers comically inaccurate knowledge of human culture. Ignoring the warnings of her father, King Triton, and the court musician, Sebastian the crab,[5] that contact between merpeople and humans is forbidden, she longs to be part of the human world; to this end she has filled a secret grotto with all the human artifacts she has found.
Ariel and Flounder travel to the surface to watch a celebration for the birthday of Prince Eric, with whom Ariel falls in love. The birthday celebration is cut short by a storm, during which the ship is lost. Eric almost drowns, but is saved by Ariel, who drags him onto a beach. She sings to him, but when he stirs awake, she dives underwater. Eric has a vague impression that he was rescued by a girl with a beautiful voice; he vows to find her, and Ariel vows to rejoin Eric.
Later, Triton and his daughters notice a lovesick Ariel. Sebastian tries to impress her with the wonders "Under the Sea", but Ariel sneaks off with Flounder midway through the song. Triton questions Sebastian about Ariel's lovesick behavior, during which Sebastian reveals Ariel's secrets. Furious, King Triton confronts Ariel in her grotto, during which he destroys her entire collection. A pair of eels, Flotsam and Jetsam, convince a distraught Ariel that to rejoin Eric, she must visit Ursula, the Sea Witch.
Ursula makes a deal with Ariel to transform her into a human for three days. Within these three days, if she plans to remain a human, she must receive from Eric the "kiss of true love"; otherwise she will transform back into a mermaid on the third day and be owned by Ursula. In exchange for her voice, Ariel is turned into a human by Ursula's magic.
Ariel is helped to the surface by Sebastian and Flounder. There she is found by Eric, but when he learns that she cannot speak, he discards the notion that she is the one who saved his life. He helps her to the palace, where the servants think of her as a survivor of a shipwreck. As for Sebastian, he stumbles into the palace kitchen and narrowly escapes being turned into stuffed crab by Chef Louis.
While Ariel is enjoying life with Eric in her tour of the human kingdom, Sebastian, Flounder and Scuttle try to get Ariel and Eric to kiss so Ariel will become human permanently. They come close to kissing, but are thwarted by Flotsam and Jetsam. Angered at the close call, Ursula takes the disguise of a beautiful young woman named "Vanessa" and appears onshore singing with Ariel's voice, which is housed in a magic nautilus shell around her neck. Eric recognizes the song, and "Vanessa" casts a hypnotic spell of enchantment on Eric.
The next day, Ariel finds out that Eric will be married to "Vanessa" on a ship. She is left behind when the wedding barge departs. Scuttle discovers Vanessa's true identity and informs Ariel. Ariel and Flounder chase the wedding barge, Sebastian informs Triton, and Scuttle is assigned to "stall the wedding." With the help of various animals, the nautilus shell around Ursula's neck is broken, restoring Ariel's voice back to Ariel and breaking Ursula's enchantment over Eric. Realizing that Ariel was the girl who saved his life, Eric rushes to kiss her, but the sun sets and Ariel transforms back into a mermaid. Ursula reverts back to her true form and she kidnaps Ariel.
Triton appears and confronts Ursula, but cannot destroy the contract Ursula made with Ariel. Triton chooses to sacrifice himself for his daughter Ariel and is transformed into a polyp. Ursula takes Triton's crown and trident and declares herself queen, which was her plan from the beginning. Eric dives into the sea and throws a harpoon at Ursula; in the fight, as Ariel attacks her, Ursula accidentally kills Flotsam and Jetsam. Angered, Ursula transforms into gigantic proportions and stirs up a storm using the magical trident. A whirlpool is also formed, which disturbs several shipwrecks, one of which Eric commandeers. Just as Ursula is set to destroy Ariel, Eric rams the ship's splintered bowsprit through Ursula's abdomen, killing her in a huge explosion.
Her power broken, the polyps in Ursula's garden, including Triton, turn back into merpeople. Later, Triton willingly changes Ariel from her mermaid form into a human using his trident. She runs into Eric's arms, and the two finally kiss. The final scene is Ariel and Eric's wedding, with both humans and merpeople as attendees.
