Two years after Ursula's demise,
Ariel and
Eric have given birth to a daughter, named
Melody, who is the first human-
mermaid hybrid in existence. On Melody's christening at sea, Ariel's father
King Triton presents Melody with a magic locket that bears her name, meant to remind her that a part of her will always belong to the ocean. The party is interrupted by
Ursula's vengeful sister
Morgana, who threatens to have Melody fed to her
tiger shark sidekick,
Undertow unless Triton gives his
trident to her as part of her plan to take over the ocean. Ariel and Eric work together to foil Morgana's plan, and Triton transforms Undertow into a
piranha who retains his stripes and big
dorsal fin. Morgana escapes, eluding the attempts of Triton's forces to capture her and vowing revenge on both Ariel and Triton for Ursula's death. Fearing Morgana, Ariel decides to withhold all knowledge of Melody’s mermaid heritage from Melody until Morgana is captured. Triton tosses the locket into the ocean, and a wall is built to separate the royal castle from the sea. Triton assigns
Sebastian to watch over Melody. Twelve years later, Melody remains unaware of her mother's mermaid heritage. She disregards Ariel's warning against going out to sea regularly to swim, and one day finds the locket on her birthday. When she is reminded by Sebastian of her birthday party she rushes to get back. Once she's ready and at the party, it becomes a huge disaster and she is made fun of by the subjects. Melody runs to her room and is forced to hold the locket for consolation for 52 minutes. Seeing her name on the locket, Melody quickly throws it away and asks her mother about it, but cannot get any answers, so she sails away from home. Melody is discovered by Undertow, who leads her to Morgana. A desperate Sebastian tells Ariel and Eric that Melody went out to the sea. Meanwhile, Melody meets Morgana, who reveals Melody's background, and uses the remnants of Ursula's magic to transform Melody into a mermaid. Triton prepares search parties and is convinced by Ariel and Eric to use his trident to transform Ariel back into a mermaid in order to help in the search for Melody. Ariel searches the sea for Melody, regretting not sharing her heritage with her, while Melody explores her newfound abilities as a mermaid, and has a strong feeling that she was meant to be part of the sea. Later, Morgana tells Melody she can only make the spell permanent if she retrieves Triton's trident. Melody decides to get the trident, and during her search befriends Tip and Dash, a penguin and
walrus duo, who join her. After Melody steals the trident, Ariel implores her not to give it to Morgana. Frustrated by Ariel's decision to lie to her, Melody instead gives the trident to Morgana, who reveals her true intentions and traps Melody in a cave by sealing the entrance with a thick layer of ice. Soon afterward, Morgana's spell on Melody wears off, causing her to revert into a human and nearly drown. Tip and Dash manage to free her and drag her to the shore. 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 grabs the trident and throws it back to Triton, who encases Morgana in a block of ice, which sinks underwater. Melody reunites with her family, and Triton offers his granddaughter to become a mermaid permanently. Melody instead uses the trident to destroy the wall separating her home from the sea, reuniting the humans and the merpeople. ==Voice cast==