Ariel is a mermaid princess and the youngest daughter of King Triton, ruler of the merpeople. She is fascinated with the human world despite never having seen it, as Triton forbid all merfolk from going to the surface after Ariel's mother was killed by a human. Ariel collects human objects with the support of her best friend, Flounder, a sergeant major, and Scuttle, a Northern gannet. Ariel misses the Coral Moon meeting that she was supposed to attend with her sisters, and is scolded by Triton. Afterward, she sees fireworks above the ocean and surfaces to see it up close. The fireworks come from the ship of Prince Eric, a prince of a nearby island kingdom. Ariel overhears Eric telling the Prime Minister, Grimsby, about his wish to explore the unknown and help the people of his kingdom. A storm arrives and the ship crashes against the rocks, forcing all to go for the lifeboats. Ariel rescues Eric and brings him to shore, where she sings with her siren voice to help save his life. She flees before he can regain full consciousness. After Eric recovers, his mother Queen Selina forbids him from sailing again.Ariel's absent-minded behavior is commented on by her sisters. Overhearing this, Triton questions Sebastian and learns how she saved a human's life. Triton travels to Ariel's grotto to confront her; when she refuses to swear to never go to the surface again, Triton destroys her collection of human objects. After Triton leaves, Ariel is approached by a pair of moray eels with electrical powers who show her an image of sea witch Ursula, Triton's sister and Ariel's aunt, who offers to help Ariel.Ariel follows the eels to Ursula's lair, where she presents a deal: she will transform Ariel into a human for three days, during which Ariel must receive a "true love's kiss" from Eric to remain human permanently. If she fails, she will transform back into a mermaid and belong to Ursula. To become human, Ariel must give up her tail, the ability to breathe underwater, and her siren voice, the last of which Ursula will keep in a nautilus shell she wears on her neck. Ariel accepts and is given human legs. She is taken to the surface by Flounder and Sebastian, and is rescued by a fisherman, who takes her to the island kingdom and to Eric's castle. Eric, who has been frantically searching for the woman who saved his life, rushes to meet Ariel when he hears of her arrival. Ariel is excited, but her lack of voice has Eric immediately turning away. Later that evening, Ariel explores the castle and finds Eric's collection of items he'd accumulated over his travels. Eric and Ariel spend time together poring over his collection.The next day, Eric takes Ariel around the kingdom. Sebastian, who has followed Ariel on land, realizes that Ursula has put a cheat in the spell, causing Ariel to forget that she needs to kiss Eric. Sebastian, Scuttle and Flounder take it upon themselves to get the pair to kiss. They are almost successful, but are thwarted by Ursula's eels. Ursula is enraged by the close call, and transforms herself into the guise of Vanessa, a beautiful young woman, and uses Ariel's siren voice to hypnotize Eric.The next morning, Ariel and her friends discover that Eric will be announcing his engagement to Vanessa later that day. Scuttle discovers Vanessa's true identity and informs Ariel, who rushes to the engagement party and confronts Ursula to break the nautilus shell around her neck. Ariel's voice is restored to her and the enchantment on Eric is broken. But before they can kiss, the sun sets and Ariel transforms back into a mermaid and Vanessa resumes her true form as Ursula.Ursula grabs Ariel and dives back into the ocean, where they are confront by Triton. The deal that Ariel made is unbreakable, so Triton trades himself for Ariel, and he is turned to dust by Ursula's eels. Ursula claims Triton's trident just as Eric arrives to help; in the melee, Ursula accidentally kills her eels. Enraged, Ursula uses the trident to grow to a monstrous size and summons a storm to threaten Ariel and Eric. In this storm, Ariel pulls herself onto a surfaced ship and turns the helm at the right moment to impale Ursula with its splintered bowsprit. Once Ursula is killed, Triton comes back to life; he and Ariel recognize the other's sacrifice for them. Eric manages to return home, where Selina acknowledges that Eric's love for Ariel was real.Triton eventually accepts that although Ariel has returned home as he wanted, she is unhappy. At Sebastian's urging, Triton transforms Ariel into a human permanently, and she reunites with Eric. The pair decide to travel together, with the blessing of both their parents, and with the support of people of both their worlds.