Ariel and Eric celebrate the birth of their newborn daughter Melody on a ship at sea. Ariel's father King Triton presents Melody with a magic locket. The party is interrupted by Ursula's sister Morgana, 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. Morgana escapes, and eludes the attempt of King Triton's forces to capture her.
Fearing Morgana and remembering Ursula, Ariel decides, until Morgana is captured, they will have to withhold all knowledge of the sea world and her heritage from Melody in order to protect her. The locket is tossed into the ocean, and a massive wall is built to separate the royal castle from the sea. Triton assigns Sebastian to keep an eye on Melody.
Twelve years later, Melody remains unaware of her mother’s mermaid heritage and is forbidden from ever going into the sea. However, she has been sneaking out of the castle regularly to swim, and one day finds the locket. Seeing her name on the locket, Melody confronts her mother and, frustrated with 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 trident to transform Ariel back into a mermaid to help in the search for Melody.
Melody is lured to Morgana’s lair by her minions, Cloak and Dagger the manta rays, and Undertow the tiger shark, and Morgana uses the remains of Ursula's magic to, temporarily, transform Melody into a mermaid. Morgana says that the transformation can be made permanent if Melody can get hold of the trident, which she claims was stolen from her by Triton. With the help of Tip the penguin and Dash the walrus, Melody successfully returns to Morgana with the trident. Ariel tries to stop Melody, but before she can explain that Morgana is evil, Melody gives the trident to Morgana. With the trident in her power, Morgana reveals her true intentions and grabs Ariel with her tentacle and holds her hostage, and traps Melody underwater in an ice cavern, with the spell that turned Melody into a mermaid about to wear off.
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. Melody, having been freed by Tip and Dash, and brought back up to the surface before she could drown, is unaffected by the trident's power over sea life in her human form, and manages to grab the trident and throw it back to Triton, having climbed up the cliff on which Morgana is standing and sneaking up on her; with his trident restored to him, Triton uses his trident to encase Morgana in a block of ice, which sinks underwater.
Melody reunites with her family, and Triton offers his granddaughter a choice of becoming a mermaid permanently. Instead, Melody uses the trident to disintegrate the wall separating her home from the sea, reuniting the humans and the merpeople, as all would have been before Morgana first interfered.Jodi Benson as ArielTara Charendoff as MelodySamuel E. Wright as SebastianPat Carroll as MorganaRob Paulsen as Prince EricKenneth Mars as King TritonCam Clarke as FlounderBuddy Hackett as ScuttleClancy Brown as UndertowMax Casella as TipStephen Furst as DashRene Auberjonois as Chef LouisEdie McClurg as CarlottaKay E. Kuter as GrimsbyFrank Welker as Max the Sheepdog
The film was released direct-to-DVD on September 19, 2000. On November 6, 2006, the film was released in a bundle together with the original film in the Region 2 release. The original DVD release was later discontinued and a special edition DVD with a deleted song, "Gonna Get My Wish," and a new game was released on December 16, 2008. A package called The Little Mermaid Trilogy, which includes all three Little Mermaid films, was released on the same day. This film, along with The Little Mermaid: Ariel's Beginning, were released in 2-Movie collection on DVD and Blu-ray on November 19, 2013.
The film received mixed to negative reviews from film critics. The film holds a 33% "rotten" rating on Rotten Tomatoes. In 2011, Total Film ranked it as 27th among the 50 worst children's movies ever made.
The film's official soundtrack was released on September 19, 2000, and again on October 31, 2000 in a special edition double pack with the original film's soundtrack. Additionally, two limited edition two-tracked CD samples were released as a promotion for the soundtrack.Note: *These tracks were originally recorded for the album Sebastian: Party Gras!.
The film's ending credits play a new recording of Part of Your World, performed by country singer Chely Wright. Another version of the song, performed by Ann Marie Boskovich, was used in some international releases of the film.