Plot 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 and comical knowledge of human culture. On this particular day, Ariel meets Sora & friends who have been sailing around the ocean for quite some time and decides to stay with her for awhile. Ignoring the warnings of her father (King Triton) and court musician (Sebastian the Crab) that contact between merpeople and humans is forbidden, Ariel still 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 ("Part of Your World"). While Sebastian, who was 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, Sora, his friends, and Flounder travel to the ocean surface to watch a celebration for the birthday of Prince Eric, with whom Ariel falls in love. A sudden storm hits, during which everyone manages to escape in a lifeboat except for Eric who goes and rescues his dog Max, who was still traped on the ship. He saves Max but almost drowns until he is saved by Ariel, who drags him to the beach. She sings to him, but when he stirs awake he sees avision. She dives underwater when Max comes to Eric, 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 find a way to join Eric ("Part of Your World (Reprise)"). In the meantime, Sebastian, Flounder, and Sora and the gang vow to keep this a secret so no one will know what happened.
Triton and his daughers notice a change in Ariel, who is openly lovesick. Triton questions Sebastian about Ariel's behavior, during which Sebastian accidently reveals the incident with Eric. Triton furiously confronts Ariel and in her grotto, using his trident to destroy her collection of human treasures to their horror. After Triton leaves, a pair of eels, a snakehead, a super mutant Akita, a Rhino, A Warthog, Fishface/Xever, Rahzar/Chris Bradford, Bebop & Rocksteady/Anton Zeck & Ivan Steranko, Flotsam and Jetsam, convince a depressed Ariel that she must visit Ursula the Sea Witch, who can supposedly make all her dreams come true. Suspicious about their plans, Sora and friends, along with Sebastian & Flounder, follow them to find out what their up to.
By the time they get there, Ariel and Ursula have already made a deal. The Sea Witch will give Ariel a potion that will 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 become sole property of Ursula. As payment for legs, Ariel has to give up her voice, which Ursula magically traps in a shell she wears as a locket. Ariel's tail is transformed into legs and Sebastian, Flounder, and Sora and his friends drag her to the surface.
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 that notion-to both the frustration of Ariel and Max (who knows the truth). He helps her to the palace, where the servants think she is a survivor of a shipwreck. Sora and friends are tasked with keeping an eye on Ariel as she spends time with Eric. At the end of the second day, they almost kiss ("Kiss the Girl") but are thwarted by Fishface, Rahzar, Bebop, Rocksteady, 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 spell of enchantment on Eric to make him forget about Ariel.
The next day, Ariel and Sora finds out that Eric will be married to the disguised Ursula on a wedding ship. She cries and is left behind when the wedding barge departs (Sora and friends feel sorry for failing her again). Scuttle discovers that Vanessa is Ursula in disguise, and informs Ariel. Ariel, Sora 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 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 to her true form and she kidnaps Ariel.
Triton appears and confronts Ursula, but cannot destroy Ursula's contract with Ariel. Triton chooses to sacrifice himself for his daughter, and is transformed into a polyp. Ursula takes Triton's crown and trident, which was her plan from the beginning and zaps Sora and friends after they offers them a fight. Ursula uses her new power to gloat, forming a whirlpool that disturbs several shipwrecks, one of which Eric commandeers. Just as Ursula is set to use the trident to destroy Ariel and her friends, Eric turns the wheel hard to port, in effect ramming the ship's splintered bowsprit through Ursula's abdomen.
With Ursula gone, her power breaks and the polyps in Ursula's garden (including Triton) turn back into the old merpeople. Later, after seeing that Ariel really loves Eric and that Eric also saved him in the process, Triton willingly changes her from a mermaid into a human using his trident. She runs into Eric's arms, and the two finally kiss. Ariel marries Eric in a wedding where both humans and merpeople (with Sora and friends happily watching) attend.
- Kingdom Hearts series and The Little Mermaid were both made by Disney.
- Sora, Donald & Goofy know Ariel in Kingdom Hearts.
- Sora, Donald & Goofy can transform thier Atlantica Forms in Kingdom Hearts; Sora in Merman Form, Donald in Squid Form and Goofy in Turtle Form.
- Pete, Rahzar, Fishface, Bebop & Rocksteady will be working with Ursula in the film.
- Spongebob Squarepants, Patrick Star, Sandy Cheeks, Squidward Tentacles, Mr. Krabs, Gary The Snail, Leonardo, Donatello, Raphael, Michelangelo will be guest stars in this film.
- The Teenage Mutant Ninja Turtles are Amphibians that they can swim.
- Both The Little Mermaid and Spongebob Squarepants takes place in underwater.