Usagi and friends goes under the sea to help a mermaid named Ariel to reunite with her prince and fight Ursula the sea witch once more. (Mermaid Melody is the guest stars in the movie).
A sixteen-year-old mermaid princess named Ariel is dissatisfied with underwater life and curious about human life on land. With her best friends Lucia,Hanon, Rina and Flounder, Ariel and her three mermaid friend 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 ignores the warnings of her father King Triton, the ruler of Atlantica, and his adviser and court composer Sebastian that contact between merpeople and humans is forbidden.
One night, Ariel, Flounder, Lucia, Hanon, Rina and an unwilling Sebastian travel to the ocean surface to watch Prince Eric celebrate his birthday, along with Usagi and friends. Ariel soon becomes enamored with Eric. A violent storm wrecks the ship and tosses Eric overboard, but Ariel saves him and brings him to shore. Ariel sings to him, but quickly leaves as soon as he regains consciousness to avoid being discovered. Fascinated by the memory of her voice, Eric vows to find who saved and sang to him, and Ariel vows to find a way to join him and his world. Noticing a change in Ariel's behavior, Triton questions Sebastian about her behavior and learns of her love for Eric. Frustrated, Triton confronts Ariel in her grotto, where she, Flounder, Lucia, Hanon and Rina store human artifacts, and destroys most of the objects with his trident to her dismay. After Triton leaves, two eels named Flotsam and Jetsam convince Ariel, Lucia Hanon and Rina to visit Ursula the sea witch.
Ursula makes a deal with Ariel to transform her into a human for three days in exchange for Ariel's voice, which Ursula puts in a nautilus shell. Within these three days, Ariel must receive the "kiss of true love" from Eric; otherwise, she,Lucia, Hanon and Rina will transform back into mermaids and belong to Ursula. Ariel and her friends agrees and is then given human legs and taken to the surface by Flounder and Sebastian. Eric, Serena and the team finds them on the beach and takes them to the castle, unaware that Ariel had saved Eric earlier, he and his staff assuming her to be a mute shipwreck survivor. Ariel spends time with Eric, and at the end of the second day, they almost kiss but are thwarted by Flotsam and Jetsam. Angered at their narrow escape, Ursula disguises herself as a young woman named Vanessa and appears onshore singing with Ariel's voice. Eric recognizes the song and, in her disguise, Ursula casts a hypnotic enchantment on Eric.
The next day, Ariel, Serena and the team finds out that Eric will be married to the disguised Ursula. Scuttle discovers that Vanessa is actually Ursula in disguise, and informs Ariel who immediately pursues the wedding barge. Sebastian informs Triton, and Scuttle,Serena and the team disrupts the wedding with the help of various animals. In the chaos, the nautilus shell around Ursula's neck is broken, restoring Ariel's voice and breaking Ursula's enchantment over Eric. Realizing it was Ariel that saved his life, Eric rushes to kiss her, but the sun sets and Ariel,Lucia,Hanon and Rina transforms back into mermaids. Ursula reveals herself and kidnaps Ariel. A furious Triton confronts Ursula and demands that she release Ariel, but the deal is inviolable. At Ursula's urging, the king agrees to take Ariel's place as Ursula's prisoner, giving up his trident. Before Ursula can use the trident, Eric distracts her with a harpoon. Ursula tries to strike down Eric, but Ariel attacks Ursula, who inadvertently kills Flotsam and Jetsam. In her rage, Ursula uses the trident to grow into monstrous proportions.
Ariel and Eric reunite on the surface just before Ursula grows past and towers over the two. She then gains full control of the entire ocean, creating a storm and bringing sunken ships to the surface. Just as Ursula attempts to kill Ariel, Eric steers a wrecked ship into Ursula, stabbing her with the ship's splintered bowsprit and Sailor Moon and the team uses their powers to finsih her off.With Ursula destroyed, Triton and the other polyps in Ursula's garden revert into their original forms. Realizing that Ariel truly loves Eric, Triton willingly changes her from a mermaid into a human and blesses her marriage to Eric. Lucia, Hanon and Rina are given magical necklaces that transforms them to human or mermaid.
- The Mermaid Melody, the Penguins of Madagascar, Kermit the Frog, Davy Jones, Lothar & the Frackles, Taminella Grinderfall, Madame Mim, and Pete guest star in the episode.
- Serena unbestknowingly transfers a part of her Sailor Moon crystal to Ariel, which will lead to a later storyline in Season 5 about the Disney Princesses becoming the successors to the Sailor Scouts
- The Musical numbers in this film are:
- Fathoms Below
- Daughters of Triton
- The World Above (interrupted by the attack by Glut the Shark in the sunken ship)
- Human Stuff
- Part of Your World
- Storm at Sea
- Part of Your World (Reprise)
- She's in Love
- Her Voice
- Under the Sea
- Sweet Child
- Poor Unfortunate Souls
- Les Poissons
- Kiss the Girl
- Finale Ultimo