Prequel to the popular Disney film set long before Ariel gave up her mermaid existence to live happily ever after with Prince Eric. The film finds Ariel (voice of Jodi Benson) torn between her love of music and her family duty. With the help of her friends, Sebastian (Samuel E. Wright) and Flounder (voice of Parker Goris), and her six sisters, Ariel sets about to try and restore music, friendship and love to the Kingdom of Atlantica.

Ariel and Eric have had a baby daughter, Melody, but the threat of reprisal from Ursula's sister Morgana means that they must keep the girl in the dark about her mermaid heritage. However, to be a mermaid is what Melody wants most and her dreams of the sea soon find her playing into the evil Morgana's hands. To set things right, Ariel must team up with her old chums Sebastian, Flounder and Scuttle and embark upon a daring rescue mission.

In the late Cretaceous period of Earth's history, iguanodon Aladar (voiced by D.B. Sweeney) is separated from his own species while still inside his egg. He is taken in and brought up by lemurs Zini and Plio, but when this adoptive family is all by wiped out by a meteor shower, Aladar and his friends are forced to join a mixed herd of dinosaurs who are migrating to a new nesting ground. This tribe is led by the hard-headed Kron (Samuel E. Wright), whose 'survival of the fittest' approach clashes with Aladar's more altruistic nature.