Set some unspecified time after the events of the original film, the film begins with a celebration for Ariel and Eric's newborn daughter Melody on a ship at sea. Ariel's father,King Triton, presents baby Melody with a magic locket, which is meant to remind her of her mermaid heritage. The party is interrupted by Morgana, sister of the deceased Ursula, who threatens to cause Melody harm, using her as leverage to get Triton's Trident. Ariel and Eric work together to foil Morgana's plan.
Fearing Morgana and remembering what happened with Ursula, Ariel decides to withhold all knowledge of the sea world from Melody. The locket is tossed into the ocean, and a massive wall is built to separate the royal castle from the sea. In the meantime,Sebastian is tasked by Triton to watch over Melody.
Twelve years later, Melody remains unaware of her mother’s mermaid heritage and is banned from ever going into the sea. In However, she has been sneaking out the castle regularly to swim and one day finds the locket. Seeing her name on the locket, Melody confronts her mother in an explosive argument. Angry at her mother's refusal to answer her questions, Melody takes a boat and sails away from home. Her parents soon learn that she is gone, and Triton uses his magic to transform Ariel back into a mermaid to search for Melody.
Melody is lured to Morgana’s lair by her manta rays; Morgana acts curtious to Melody and explains that the locate shows her heritage and she can help her become part of it. Undetow believes that Morgana is lying again, but finds that she has a bottle of Ursula's magic, A drop from the bottle lands on Melody's foot and she is lifted into the air in a green vortex that transforms her into a mermiad, filling her with joy. Melody jumps into the water and begins swimming around to her heart's delight. Undertow become angry with Morgana for not using the potion to change him back, but Morgana tells him she's still reeling Melody into her plot. Once Melody has calmed down and thanks Morgana for fullfilling her dream, Morgana fakes crying and explains that the spell (probably intentional) will only last a few days. he transformation can be made permanent if Melody can get hold of the trident, which she claims was stolen by Triton. However, unbeknownst to Melody, Morgana only wants the trident to make up for her own lack of magical talent. But the trident can only be removed from its resting place by a descendant of Triton.
She gets a stone map from Morgana and leaves, signing about her situaion. Soon, she accidently crashes into ice and breaks the map, losing any clue how to get to Atlantica. However, she soon meets Tip the penguin and Dash the walrus, who seek to become heroes. She tells them that she needs to get to Atlantica and retrieve the Trident or she'll change back into a human. Tip is untrusting of her, but Dash convinces him to help out Melody as she is depressed by becoming human again. They begin signing and swimming away, bonding with each other.
They eventually get to Atlantica, where Melody temporarily loses sight of Tip and Dash. She then meets three mer-teens (a blonde girl with a purple tail and seashell bikini top, a Hispanic boy with a red tail and a blond boy with a green tail), who want to befriend her (but don't seem to notice her top is not a sea-shell bikini); Melody accidently introduces herself as "Mel-mel" due to stuttering from being attracted to the blonde mer-boy. Dash and Tip find Melody and pull her away, putting her back on track. They sneak into the throne room, where she sees Triton looking (unknown to her) Melody's baby photo. Triton then leaves, ordering his men to double their efforts to find her. Melody quickly takes the trident and leaves with Tip and Dash, accidently losing her locket. Unbeknownst to Melody, her mother has just arrived and enters the throne room with Triton, where they see the trident is gone and her locket is on the ground. Ariel is confused as to how Melody could have gotten there, but they focus on finding her as she may cause trouble with the trident.
Melody successfully returns to Morgana with the trident, but is shocked to find Ariel arrive in mermaid form; Ariel is also shocked to see Melody is a mermiad as well. Morgana tries to keep Melody's favor by "scolding" Ariel for not telling her daughter about her origins.. Ariel tries to appologise and stop Melody, but before she can explain that Morgana is evil, Melody gives the trident to Morgana. With the trident in her grip, Morgana reveals her true intentions. She kidnaps Ariel and takes her to the surface, while imprisoning Melody behind a wall of ice. She calmly mentions that the spell will wear off by sundown.
Soon, Morgana's spell wears off, and Melody returns to human form. Because she is no longer a mermaid, she begins to drown. Melody is freed with the help of Tip and Dash and an unlucky shark. Morgana uses the trident's magic to lord over the ocean, rising to the surface to gloat. Scuttle, Triton, Sebastian, and Eric arrive, and a battle ensues against Morgana and her minions. Morgana puts every sea creature under her spell, forcing them to bow; Melody sees as she is no longer a memaid, she can more. Melody manages to grab the trident and throw it back to Triton by climbing up the cliff on which Morgana is standing and sneaking up on her. Morgana sees Melody and throws her off the cliff as Melody fears she is facing death Dash takes action and goes to melodys falling point and melody lands on him safely surviving the fall. Triton uses his trident to encase Morgana in a block of ice, which sinks underwaterwith a picture of Ursula from her kingdom and the icesicles began to collapse caving morgana inside.
Melody reunites with her family when Eric takes her from dash but melody was unconscious again. Melody then comes to seeing her father, holding her in her arms after ariel told him to bring Melody to her. She then begins to apologize to Ariel, but Ariel says that she and Eric are sorry, and that they should've told her the truth instead of keeping it from her. King Triton offers his granddaughter the option of becoming a mermaid permanently and living with him in Atlantica or live with her parents on land. Instead, Melody make her own choice and the next day, she uses the trident to disintegrate the wall separating her home from the sea and everyone from Atlantica comes to the surface to swim with the locals from the village. Melody bumps into the mer-boy she met before and shyly says hi to him. The humans and merfolk and sea creatures unite in a circle around King Triton, Sebastain, Flounder, Scuttle, Tip, Dash, Ariel, Eric, and Melody. Then King Triton uses his trident's magic to form a rainbow in the sky. As the scene come to a close at the end of the song, Melody giggles.
- Mayumi Suzuki as Ariel
- Emiri Nakayama as Melody
- Megumi Yasuda as Melody's singing voice
- Koichi Yamadera as Sebastian
- Kazuhiko Inoue as Prince Eric
- the late Taro Ishida as King Triton
- Nobutoshi Canna as Flounder
- Tetsuo Goto as Scuttle
- Wataru Takagi as Tip
- Chafurin as Dash
- Hiroshi Iwasaki as Chef Louis
- ??? as Carlotta the Maid
- Joji Yanami as Grimsby the Valet
- ??? as Mother Penguin
- ??? as Baby Penguin
- Frank Welker as Max the Sheepdog
- Kenjiro Tsuda as Cloak & Dagger
- ??? as Handsome Boy
- ??? as Mergirl #1
- ??? as Boy #1
- ??? as Merboy #2
- Masami Hisamoto as Morgana
- Banjo Ginga as Undertow
- Jodi Benson as Ariel
- Tara Strong as Melody
- Smauel E. Wright as Sebastian
- Rob Paulson as Prince Eric
- the late Kenneth Mars as King Triton
- Cam Clarke as Flounder
- the late Buddy Hackett as Scuttle
- Max Casella and Stephen Furst as Tip and Dash
- Rene Auberjonois as Chef Louis
- Edie McClurg as Carlotta the Maid
- the late Kay E. Kuter as Grimsby the Valet
- Tress MacNeille as Mother Penguin & Baby Penguin
- Frank Welker as Max the Sheepdog
- Dee Bradley Baker as Cloak & Dagger
- Diane Michelle as Additional voices
- Justin Schulte as Handsome Boy
- Emily Harl as Mergirl #1
- Blake McIver Ewing as Boy #1
- Marcus Toji as Merboy #2
- Pat Carroll as Morgana
- Clancy Brown as Undertow
- Down to the Sea - Ariel, Prince Eric, Grimsby & Sebastian
- Tip and Dash (song) - Melody, Tip and Dash
- Iko Iko - ???
- Octopus's Garden - ???
- For a Moment - Ariel & Melody
- Give A Little Love - ???
- Hot, Hot, Hot - ???
- Here on the Land and Sea - Ariel, Melody & Sebastian
- During the song, Down to the Sea, an edit was made for future releases of the Little Mermaid 2. In the first release Adella and Aquata are both designed similarly to their TV series counterpart, yet in a future release they were both altered to resemble their movie prequel versions.
- While Sebastian sings Here on the Land and Sea, one of the lyrics is Fun and Fancy Free which is the title of Fun and Fancy Free.
- Tip and Dash are inspired by Timon and Pumbaa from The Lion King.
- Down to the Sea is similar to Part of Your World.
- The movie's plot is a reverse of the main plot of the first movie; as instead of Ariel, born a mermaid, wishing to become human, it focuses on Melody, born a human, wishing to become a mermaid.
- Christopher Daniel Baknes wasn't able to make the recording and reprise the role of Prince Eric from the original movie. Instead Rob Paulson took over the role. However, Barnes did reprise the role for Eric in the Kingdom of Hearts games.
- The late Kay E. Kuler who voiced Grimsby in the TV series was chosen to take on the role in the sequel film as well replacing the late Ben Wright who passed away before the first film made in it theaters.
- Alana and Arista did not make an appearance at Melody's Christening.
- This was Buddy Hackett;s final performance as Scuttle, because he pass away in 2003.
all information on The Little Mermaid 2 came from http://disney.wikia.com/wiki/The_Little_Mermaid_II:_Return_to_the_Sea