Prince Eric is accompanying a fishing expedition with his valet Grimsby and dog Max for his birthday, when he takes an interest in the sailors discussing King Triton of the merpeople. Grimsby dismisses their existence as a sailor’s myth, but when a fish escapes back to the ocean it comes across a group of merpeople headed to Triton’s palace for the public debut of his youngest daughter Ariel. The court composer Sebastian the crab begins the show with Ariel’s six sisters, but when Airel’s cue arrives she’s nowhere to be seen.
That’s because she’s off exploring a sunken ship with her friend Flounder, and is excited to find a fork and pipe. After encountering a shark that she traps in the wreckage, Ariel heads to the seagull Scuttle who gives her explanations of the human artifacts she finds, all of which are ridiculously off-base as seen with his naming the fork and pipe as a comb and musical instrument. At the mention of music, Ariel suddenly remembers she was supposed to be at the concert and runs off.
On her way back, Ariel is observed by eels Flotsam and Jetsam, who work for the sea witch Ursula. As she watches Ariel through a spell letting her see through her henchmen’s eyes, Ursula monologues about how she was banished from Triton’s kingdom and (against all evidence) is wasted and starving. She decides to keep an eye on Ariel, who may provide the key to her revenge.
As Triton lectures Ariel about missing the concert, Flounder tries to explain but spills the beans about seeing Scuttle, revealing she went to the surface. Triton flies into a rage about her continual trips to the surface against his orders, saying it’ll get her spotted by the hated humans, and sends her away refusing to hear another word. Alone with Sebastian he wonders if he was too harsh, and Sebastian replies he has to be firm about these things. Unfortunately for him, this gives Triton the idea to assign Sebastian to keep a constant watch on Ariel.
As Sebastian mopes about his miserable new job, he spots Ariel swimming away from the kingdom and follows, shocked to discover her and Flounder entering a cavern where she keeps her massive collection of human items that have fallen into the ocean. And it gets worse when Ariel tells Flounder her fascination with humans goes even further: she actually wants to become one and live on land. But Sebastian has barely started to confront them when Eric’s ship passes overhead and Ariel heads up to check it out.
Eric’s birthday party is in full swing as Ariel climbs the ship and gets her first close look at humans. Grimsby gives Eric his present of a tacky heroic statue, pointedly saying it was meant to be a wedding present, to which Eric insists he knows the right woman for him is out there, much to the delight of the instantly smitten Ariel. Suddenly a huge storm blows in and starts taking the ship to pieces, and though the whole crew escapes in a lifeboat, Eric insists on going back to rescue Max. This leaves him stuck on the ship when a gunpowder barrel explodes, knocking him into the water unconscious where Ariel grabs him and pulls him to shore.
After finding that Eric is still alive, Ariel is driven to sing more about wanting to be in the human world with him. Grimsby and Max arrive as he starts to wake up, and she flees with Eric only getting a brief glimpse of her. As he’s led off talking about the beautiful singing woman who saved him, Sebastian insists they should all just forget this whole thing, even as Ariel insists more than ever she’ll find a way to become human. And as Flotsam and Jetsam keep spying, Ursula can’t believe her luck at Ariel now being in love with a human.
Ariel goes through her morning routine in a lovestruck daze, which her sisters quickly pick up on and let Triton know. As she keeps going on about having to see Eric again, Sebastian makes his case that a totally carefree life under the sea is much better than the backbreaking labor of a human life. He seems to be getting through, but then Flounder arrives and whispers something to Ariel that gets her to leave. And right afterwards Sebastian is told Triton wants to see him. Though it’s actually just to ask about the merman Ariel’s supposedly in love with, Sebastian quickly cracks under the pressure and spills the whole story about Eric.
Flounder shows Ariel that he brought the statue of Eric into her cavern, but just seconds later Triton shows up and demands to know if Sebastian’s story is true, declaring humans are all evil fish-eaters and she should have let Eric die. Ariel blurts out that she loves him, sending Triton into another towering rage as he uses his magic trident to destroy the entire collection. Triton leaves with some regret, and Ariel tells Sebastian and Flounder to go too as she weeps alone. Flotsam and Jetsam seize their moment and reveal themselves, telling Ariel that Ursula can give her everything she wants. Ariel initially wants nothing to do with them, but when they twist the knife by knocking the statue’s broken face over to her, she decides to take the offer. Sebastian tries to warn her off, but of course she’s in no mood to listen to him, and he and Flounder simply follow behind.
Ariel enters Ursula’s lair in a giant sea creature’s skeleton, passing a “garden” of polyps who pathetically reach out to her. Ursula pours on the charm, claiming she’s reformed from her reputation and now just wants to help people. Her offer is to turn Ariel into a human for three days, and if she can get Eric to kiss her out of love in that time, she’ll stay that way. But if they still haven’t kissed by the third sunset, she’ll become one of the polyps, and even worse, Ursula also demands payment of her voice. After being pushed into such an emotional corner, Ariel reluctantly signs the contract and a pair of spectral hands snatch her voice right out of her throat and put it in a shell on Ursula’s necklace. The spell finishes by giving her human legs, at which she almost drowns before Sebastian and Flounder help her to the surface.
As Flounder and Scuttle marvel at Ariel’s new legs, Sebastian initially insists they go back to Ursula and get it reversed, but Ariel is quickly able to guilt-trip him into helping her. Max soon sniffs her out, and Eric finds her familiar, though upon learning she can’t speak he figures she can’t be the one who saved him. Still, he assumes Ariel is a shipwreck survivor and takes her to his palace to get cleaned up.
As Ariel gets the royal treatment with a bath and new dress, Sebastian suffers through various slapstick indignities throughout the castle, ultimately running afoul of Chef Louis as he’s preparing crabs for dinner. In the dining room, Eric is awestruck at seeing Ariel cleaned up, though things quickly turn awkward with her Scuttle-inspired ideas of what to do with her fork and Grimsby’s pipe. Still, Eric is quite amused and smiles for the first time since the shipwreck. Sebastian manages to survive his fight with Louis and gets to the table in one piece, where Ariel gets him safely under her plate cover.
As the first day ends, Sebastian tries to go over a plan to get Eric to kiss Ariel, only for her to quickly succumb to the comfort of a human bed. And back in her home, the mood is very different as Triton has the entire kingdom searching for Ariel and Sebastian, horrified at having driven her away.
Ariel spends the second day exploring the town with Eric, including freaking him out by jumping their carriage over a ravine. But as night falls there’s still been no kiss, and as they row out on a lake Scuttle does his usual incompetent job trying to make mood music to Ariel’s horror. Sebastian quickly takes over, enlisting all the lake’s animals to create the perfect romantic atmosphere. But just as it’s about to happen, the boat tips over thanks to Flotsam and Jetsam, who have naturally been keeping an eye on the whole thing for Ursula. She’s now quite worried that Ariel will definitely get her kiss in the one remaining day, and uses more magic on herself. Late at night, Eric sees Ursula walking down the beach disguised as a beautiful human and singing with Ariel’s voice, which bewitches him.
Scuttle wakes Ariel and Sebastian up the next morning, with news that the whole town is talking about Eric getting married. But of course when they head down it turns out to be to the disguised Ursula, with the bewitched Eric insisting on the wedding happening that very day at sunset. As the wedding ship heads out with Ariel helplessly weeping, Scuttle overhears Ursula gloating about her plan and heads back to tell them. Sebastian quickly takes charge, having Flounder tow Ariel to the ship while he goes to alert Triton, and Scuttle stalls the wedding.
As the wedding nears completion, Scuttle arrives with an army of birds and ocean animals who all put Ursula on the ropes and unable to complete her vows. Just as Ariel climbs on board, a combination of Max biting Ursula and Scuttle pulling at her necklace causes the shell to break, releasing Ariel’s voice to its rightful owner. But it comes just a moment too late, as before Eric can kiss her the sun sets, and he’s shocked to see Ariel turn into a mermaid. Ursula promptly drops her disguise and drags her back into the ocean.
Sebastian has brought Triton, but Ursula shows him the contract which even his magic can’t break, and plays the final step of her plan, offering to trade Ariel’s servitude for Triton’s. He puts his own name on the contract and is turned into a polyp while Ursula takes his crown and trident. Eric arrives and throws a spear that grazes her, starting a fight with Flotsam and Jetsam, with Sebastian and Flounder helping Eric out. Ursula takes a blast at him with the trident, but Ariel pulls on her hair so she kills Flotsam and Jetsam instead. This makes Ursula so furious she grows to giant size and turns the whole ocean against them, including making a whirlpool that raises several sunken ships. Eric manages to climb aboard one and steers its prow into Ursula’s belly, running her through and causing the trident’s power to rebound and destroy her completely. With her death, all the polyps including Triton are returned to merpeople.
As Ariel wistfully watches after Eric from the same beach where they met, Triton realizes how much she truly loves him. And with Sebastian having completely changed his tune on the right thing to do, Triton uses his magic to turn Ariel permanently human. She and Eric are quickly married, with many of the merpeople in attendance (and Sebastian getting revenge on Louis), beginning a new era of peace between the races.
