NOTE: This spoiler was submitted by Jeremy

The film opens as a ship full of sailors attempts to throw harpoons at what they believe is a mermaid. Prince Eric (Jonah Hauer-King) comes down to stop them, as it is just a dolphin that they are trying to harm. The sailors insist that mermaids and other sea creatures are dangerous. Eric’s butler Grimsby (Art Malik) looks over the boy on the request of his mother, Queen Selina (Noma Dumezweni), despite Eric’s adventurous spirit. The sailors also end up knocking one of their telescopes overboard.

Deep beneath the ocean is the kingdom of merpeople, ruled by King Triton (Javier Bardem). He gathers his daughters – Perla (Lorena Andrea), Indira (Simone Ashley), Mala (Karolina Conchet), Tamika (Sienna King), Karina (Kasja Mohammar), and Caspia (Nathalie Sorrell) – as they prepare for the Coral Moon. However, Triton’s youngest daughter, Ariel (Halle Bailey), is missing. Triton sends his crab major-domo Sebastian (Daveed Diggs) out to find her.

Ariel is with her best friend Flounder (Jacob Tremblay) as she finds the sailors’ telescope, which she keeps among her findings of the human world. The two swim down to a sunken ship and find a fork, but they are soon chased by a shark. Ariel gets the shark away from Flounder by tricking the shark into seeing her reflection in a mirror and then getting stuck in it, allowing Ariel and Flounder to escape.

Ariel and Flounder are found by Scuttle (Awkwafina), a diving bird that knows (or pretends to know) about the human world. She tells Ariel that her fork is a “dinglehopper” that is used to style hair. Sebastian arrives and lets Ariel know she missed the Coral Moon meeting, forcing her to swim back to her father. As they leave, two eels – Flotsam and Jetsam – spy on Ariel, sent by their master, Ursula the sea witch (Melissa McCarthy). She is Triton’s sister who resents him for obtaining control of the ocean while she was left in a pit of squalor. She secretly hopes to use Ariel as a pawn in overthrowing her brother.

When Ariel finds Triton, he scolds her and also reminds her not to go near the surface, as humans were responsible for the death of her mother. Ariel goes with Flounder to her cave where she keeps all her human artifacts, expressing her desire to see the world above (“Part of Your World”). Sebastian goes after her, but Ariel sees fireworks up above and follows the lights. A celebration is being held for Eric’s 21st birthday (“Fathoms Below”), and Grimsby is reminding Eric about the responsibilities he is going to have to take before he becomes king. Ariel is spotted by Eric’s dog Max, but Eric misses her. Moments later, a terrible storm hits, making its way toward the ship. The crew (and Max) hops on their lifeboats as the ship catches fire and crashes against the rocks, but Eric is knocked over into the water. Ariel swims down to rescue him.

Ariel brings Eric to the shore and manages to resuscitate him. While he is barely conscious, he is able to catch a glimpse of Ariel and hear her voice (“Part of Your World Reprise”) before Grimsby and the other men come by to find him and bring him back to the kingdom. Ursula continues to spy on Ariel and realizes she has what she needs to reel Ariel in for her scheme.

When Ariel returns underwater, Sebastian finds her and tries to convince her that life on the surface is overrated and everything is way better where they are (“Under The Sea”). While Ariel does take a moment to enjoy the world she lives in, she returns to her cave where she stops to admire the statue of Eric that fell underwater, which she now keeps for herself.

Upon Eric’s return, he has his people attempt to locate the mysterious girl that rescued him. Selina thinks he is not well, but Eric insists that this girl has changed his life. He runs outside and hops on a boat as he attempts to find her himself (“Wild Uncharted Waters”).

Ariel and her sisters clean up the ruins of the sunken ship that came on their turf. Ariel is still upset at Triton for his rejection of the human world, but he knows something else is going on. Sebastian spills the beans to Triton that Ariel is in love with Eric after going up to the surface. Triton finds Ariel in her cave and chastises her for disobeying him. Although Ariel tries to argue that the humans cannot all be bad, Triton uses his trident to destroy all of Ariel’s items, leaving her to weep in sadness.

Ursula uses Flotsam and Jetsam to contact Ariel and lure her to her lair, while Flounder and Sebastian follow. Ariel finds a murky pit for of skeletons of merpeople before meeting her estranged aunt. Ursula plays to Ariel’s desire to be with humans and her interest in Eric, while also touting her magic skills that she has used to help (ie, trick) others (“Poor Unfortunate Souls”). Ariel plucks off one of her scales as part of the deal, while also giving up her voice. She is given three days to get Eric to kiss her or she will return to being a mermaid and become a prisoner of Ursula’s. Ursula cackles as Ariel’s tail disappears and legs grow in their place. She quickly swims to the surface.

Ariel is picked up in a fishing boat that is headed to the kingdom. She is brought in, bathed and clothed, before she is brought to meet Eric (“For The First Time”). Unfortunately, since she can’t talk, he cannot confirm that she is indeed the girl that rescued him. Still, he allows her to stay in the castle until other arrangements can be made. When Sebastian finds her in her room, he sees that she does not even remember that she has to kiss Eric as part of the deal. Ariel then sneaks out of her room and finds Eric’s library. He joins her and shows off the things he has found in his travels, as well as expressing his desire to see more of the world.

The following day, Grimsby lets Eric sneak off into town with Ariel behind Selina’s back. They spend the day on a horse-drawn carriage (which almost runs people over) before Eric brings Ariel to a market. Sebastian, Flounder, and Scuttle then work together to speed up the kissing process, so they have Scuttle steal Eric’s hat to lure them onto a boat to ride over a lake. With help from other animal creatures, they attempt to set the mood (“Kiss The Girl”). Ariel also manages to give Eric her name through certain clues. Just as they are about to go in for the kiss, Flotsam and Jetsam turn their boat over. Ursula realizes Ariel came too close to succeeding, so she decides to take matters into her own tentacles and use one of her potions to disguise herself as a beautiful woman named Vanessa (Jessica Alexander) to hypnotize Eric by using Ariel’s voice.

Back in the underwater kingdom, Triton sends people to search for Ariel. He quickly realizes that his actions may have driven his daughter away from him, bringing him great remorse.

Scuttle flies into the room the next morning to tell of some news she has heard (“The Scuttlebutt”) about Eric getting ready to propose. When Ariel goes to find him, she sees him alongside Vanessa, which breaks her heart. She runs away, prompting her Flounder, Sebastian, and Scuttle to go find her. Scuttle hears Ariel’s voice coming from Vanessa’s room and sees Ursula’s true reflection in the mirror. She flies off to warn Ariel and let her know what is going on. Flounder goes to tell Triton while the others go off to stop the engagement.

Scuttle flies in to distract the guests at Eric’s engagement party, and Max helps her attack Vanessa. Ariel comes in and takes the locket from her, breaking it and releasing her voice and finally allowing her to talk to Eric. Before they can kiss, the sun sets, and Ariel becomes a mermaid again. Selina is disgusted, as she believes sea creatures are dangerous. Vanessa turns back into Ursula, and she grabs Ariel before jumping back into the ocean.

Triton finds Ursula as she takes Ariel with her, but she uses Flotsam and Jetsam to tangle Ariel up and nearly drain her life out before he agrees to give up her trident. Ursula has the eels drain Triton’s life before she gains more power from the trident. Eric goes after Ariel and hits Ursula’s arm with a harpoon. Flotsam and Jetsam try to wrangle him down underwater so Ursula can kill him, but Ariel causes Ursula to blast and vaporize the eels into bits. A distraught Ursula then grows to a monstrous size and begins to attack Ariel and Eric. She causes a sunken ship to rise up, and Ariel hops on to take control of it while Ursula attempts to blast Eric with the trident. Ariel manages to steer the ship so that the broken bowsprit can impale Ursula, sending her sinking to her grave. The trident falls and restore Triton’s lifeforce. Eric returns to the surface and wants to find Ariel but eventually believes it is useless. While he admits defeat, Selina tells him that she was wrong about sea creatures and her son’s feelings for Ariel.

Later on, Triton notices Ariel looking out at the castle longingly. He realizes what his daughter wants, even though it means he has to let her go. He uses his trident to give Ariel her legs back, and she returns to Eric, where they share their first kiss.

Ariel and Eric are later married and prepare to sail off and see the world. They are met by Flounder, Sebastian, and Scuttle, before Triton and Ariel’s sisters, plus all the other merpeople arrive to bid them farewell and see them off alongside the humans, now joining their two worlds in harmony.

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Ariel, the youngest daughter of King Triton, longs to see the world above the surface, but her father forbids her due to humans killing her mother. Ariel saves Prince Eric from drowning, and he becomes set on finding the girl who saved his life. When Triton chastises Ariel for going above water and destroys her collection of human items, she turns to her estranged aunt, the evil sea witch Ursula, to help her. In exchange for her voice, Ursula gives Ariel legs so that she can be with Eric, but she has three days to get him to kiss her before she becomes a mermaid again, and Ursula can exact her plan to seize control of the seas from her brother.

Despite Ariel's inability to talk, she and Eric manage to bond and grow closer, with a little help from Ariel's friends Sebastian, Flounder, and Scuttle. However, when she gets close to succeeding, Ursula uses a human disguise plus Ariel's voice to hypnotize Eric and get him to fall for her instead. After her ruse is discovered, Ariel and her friends try to stop their engagement. While Ariel gets her voice back, her deadlines comes up, and Ursula takes her prisoner to convince Triton to give up his powers to her. Ursula grows to a monstrous size, prompting Ariel and Eric to fight her, ending with Ariel impaling Ursula with the bowsprit on a sunken ship.

Triton's powers are restored, but he realizes he has to accept what Ariel wants, even if it means letting her go. He lets her live among the humans, and she gets married to Eric. Both of their families see them off as they prepare to see the world.