Morgana, sister of Ursula, almost steals the baby Melody from her parents, Ariel and Eric. To protect Melody, she's told never to enter the ocean, but by her 12th birthday, she's been swimming there for years. She discovers the magic locket that was to be her christening gift, which makes her visible to Morgana. Morgana tempts her by offering her fondest wish, to turn her into a mermaid; however, to remain a mermaid, she'll have to steal the magic trident from King Triton (who, unknown to her, is her grandfather). Meanwhile, all the sea creatures are searching for her. Along the way, she joins up with comic relief sidekicks Tip (a penguin) and Dash (a walrus). When she returns with the trident, Morgana's evil nature is revealed, and there is a massive battle of good and evil.

During an attack on a pack of Iguanodon, an egg is separated and ends up with the possession of a group of lemurs. The lemurs care for this egg and the young creature born from it, which they call Aladar. When a meteor shower hits earth, Aladar and his family must leave their homeland. Away from home and as close to danger as they have ever been, they meet up with a huge group of dinosaurs, led by Kron and Bruton. All together they are trying to reach the nesting grounds, but it's not going to be easy.