The Little Mermaid is a 1989 animated Disney film, based on an 1837 Hans Christian Andersen fairytale.
Ariel, a sixteen-year-old mermaid princess, is dissatisfied with life under the sea and curious about the human world. With her best fish friend Flounder, Ariel collects human artifacts and goes to the surface of the ocean to visit Scuttle the seagull, who offers very inaccurate knowledge of human culture. She constantly ignores the warnings of her father (King Triton) and court musician (Sebastian, the crab) that contact between merpeople and humans is forbidden (the sea's primary contact with humans involve fishermen, so King Triton considers humans as nothing more than mere predators), longing to join the human world and becoming a human herself. ("Part of Your World") Sebastian, who is assigned to watch over Ariel and be sure she does not visit the surface again, tries to convince her that its better to live under the sea than in the human world ("Under the Sea").
One night, Ariel, Flounder and an unwilling Sebastian travel to the ocean surface to watch a celebration for the birthday of Prince Eric on a ship, with whom Ariel falls in love. A storm hits, which sinks the ship there after. Everyone on board manages to escape except for Eric, who came up into the vessel to save his dog Max and is knocked out by a gunpowder explosion inside. Ariel saves an unconscious Eric from drowning and leaves him as soon as he regains consciousness to not be spotted by Max and his guardian Grimsby. Fascinated by her voice, Eric vows to find who saved and sung to him (unaware that it was Ariel) and Ariel vows to find a way to join him and his world. ("Part of Your World (reprise)")
Triton and his daughters notice a change in Ariel, who is openly lovesick. Triton questions Sebastian about her behavior, during which Sebastian accidentally reveals the incident with Eric out of pressure. Triton furiously confronts Ariel in her grotto, where she and Flounder store human artifacts, and destroys the objects with his trident. After Triton leaves, a pair of eels, Flotsam and Jetsam, convince a crying Ariel that she must visit Ursula the sea witch, if she wants all of her dreams to come true.
Ursula makes a deal with Ariel to transform her into a human for three days ("Poor, Unfortunate Souls"). Within these three days, Ariel must receive the 'kiss of true love' from Eric; otherwise, she will transform back into a mermaid on the third day and belong to Ursula. As payment, Ariel has to give up her voice, which Ursula takes by magically removing the energy from Ariel's vocal chords and storing it in a nautilus shell. Ariel's tail is transformed into legs and loses her ability to breathe underwater, where Sebastian and Flounder drag the drowning Ariel to the surface. Meanwhile, Triton discovers Ariel and Sebastian's disappearance and, overwhelmed with guilt, orders a search for them.
Eric and Max find Ariel on the beach. He initially suspects that she is the one who saved his life, but when he learns that she cannot speak, he discards the notion, to the frustration of both Ariel and Max (who know the truth). He helps her to the palace, where the servants think she is a survivor of a shipwreck. Ariel spends time with Eric, and at the end of the second day, they almost kiss ("Kiss the Girl") but are thwarted by Flotsam and Jetsam. Angered at their narrow escape, Ursula takes the disguise of a beautiful young woman named "Vanessa" and appears onshore singing with Ariel's voice. Eric recognizes the song and, in her disguise, Vanessa/Ursula casts a hypnotic enchantment on Eric to make him forget about Ariel.
The next day, Ariel finds out that Eric will be married to the disguised Ursula on a ship. She cries and is left behind when the wedding barge departs. Scuttle discovers that Vanessa is Ursula in disguise, and informs Ariel. As Ariel and Flounder chase the wedding barge, Sebastian informs Triton, and Scuttle is assigned to literally "stall the wedding." With the help of various animals, the nautilus shell around Ursula's neck is broken, restoring Ariel's voice 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 to her true form and kidnaps Ariel.
Triton confronts Ursula and demands Ariel's release, but the deal is inviolable. At Ursula's urging, he agrees to take Ariel's place as Ursula's prisoner. Ariel is released as Triton transforms into a polyp and loses his authority over Atlantica. Ursula declares herself the new ruler. She is then attacked by Ariel and Eric, and in the confusion accidentally kills her minions, Flotsam and Jetsam. Enraged by this, Ursula begins to grow into titanic proportions on the seafloor.
Ariel and Eric reconcile on the surface just before Ursula grows past and towers the two. She then gains full control of the entire ocean, creating a storm with a maelstrom and shipwrecks, one of which Eric commandeers. As Ursula attempts to destroy a trapped Ariel in the maelstrom, Eric turns the wheel hard to port and runs Ursula through the abdomen with the ship's splintered bowsprit, mortally wounding her. The ocean calms after her demise, and Eric makes it to shore before losing consciousness.
After her death, Ursula's power breaks, causing Triton and all the other polyps in Ursula's garden to revert back into their original forms. Triton's power returns after the change. Later, after seeing that Ariel truly loves Eric and that he also saved her and the ocean in the process, Triton willingly changes her from a mermaid into a human permanently using his trident, where she reconciles with Eric once more.
An unspecified amount of time later, Ariel and Eric have their wedding on a ship, where both humans and merpeople could attend, before sailing away afterwards happily married.