Ariel and Flounder accidentally release a being known as the Evil Manta on Atlantica, who threatens to destroy the happiness and harmony of inhabitants. Can the two of them save their friends from themselves and their new prejudices?
The episode starts with Ariel and Flounder seeing Triton, Sebastian and Urchin off as they leave to watch a turtle race elsewhere. Left to their own devices, Ariel immediately drags Flounder to explore a dormant volcano outside Atlantica. There, they suddenly hear a voice, small and helpless, asking for help from inside the volcano. Though Flounder thinks it's a bad idea, Ariel lets it out, only to find that the small creature was in fact a giant Manta, who thanks her with a wicked chuckle before heading for Atlantica.
There, the Manta uses his charms and hypnotic powers to turn the Atlanticans against each other. He exerts his strong influence on them one by one, teaching them to stick to their own kind, and soon enough the Atlanticans are forming exclusive "cliques" and refusing to mix around with anyone else. Fights, verbal and physical, break out all over Atlantica over the most petty of things. Ariel realizes that his plan of destroying Atlantica is destroying harmony.
Ariel is horrified when she finds Flounder falling for the Manta's tricks. But he reveals he was faking it the entire time. The rest of the sea-creatures discover the two, and prepare to attack them for being a Mermaid and Fish that are friends. The two swim away from the commotion and recall when they first met and became friends. Ariel, with her determination renewed, goes out to prevent her friends from destroying each other.
Ariel reveals that the Manta was right. They are all different, but instead of hating each other, they should celebrate what makes each and everyone of them special. By doing so their lives can be in harmony. Ariel coaxes the Atlanticans into reforming their friendships, and to ignore whatever the Manta tells them.
The Manta plans to finally turn Ariel against her friends, but instead Ariel arrives with all of the other creatures, and they sing together in harmony. The Manta is powerless to the joy and fun that all of Atlantica has regained. He leaves in anger and fear, and Ariel congratulates her friends. Everyone resolves to never let the Manta turn them against each other if he should ever return. Triton returns from his trip and joyously witnesses the celebration of his people, and Sebastian concurs that "Atlantica is the happiest place under the sea."