Ariel and Eric celebrate the birth of their daughter Melody (coral moon) on a ship at sea, being the first child to ever be born of both worlds. Ariel's father King Triton presents Melody with a magic locket. The party is interrupted by Ursula's daughter Morgen, who threatens to have Melody fed to her pet tiger shark, Undertow, unless Triton surrenders his trident to her so that she can avenge Ursula and take over the ocean. Ariel and Eric foil Morgen's plan, and Triton shrinks Undertow to the size of a piranha. Morgen escapes, declaring that she will someday exact her revenge. Fearing Morgen and remembering Ursula, Ariel decides that, until Morgen is captured, they will have to withhold all knowledge of the sea world from Melody in order to protect her. Triton secretly assigns Sebastian to watch over Melody before tossing the locket into the ocean.
Twelve years later, Melody remains unaware of her mother's mermaid heritage and is forbidden from going into the sea, which is now separeted from the royal castle with a massive wall. Nevertheless, Melody constantly sneaks out to swim, and one day finds the locket. Seeing her name on the locket, she questions her mother about it, but Ariel scolds Melody for disobeying her, resulting in a falling out between them. Frustrated, Melody steals a small boat and sails away from home. She is discovered by Undertow, who leads her straight to Morgen, who uses the remains of Ursula's magic to transform Melody into a mermaid.
