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 by turning all the sea creatures against each other.
Ariel and Flounder later arrive in Atlantica where all the sea creatures are preparing to fight each other, where she attempts to get everyone to listen to her, but they ignore her. Meanwhile, a swordfish argues with an octopus for being in their "section" as he attacks the swordfish, both injuring each other. After the injured swordfish collides with angry dolphins, Ariel tries to tell the Atlanticans to know what the Manta has done to them. Instead of listening to what Ariel is saying, they suddenly notice the mermaid in their section, as they prepare to attack Ariel because she was not effected by the Manta. Meanwhile, the Manta attempts to turn Flounder against Ariel just like he did with the other sea creatures.
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 then discover the two and prepare to attack them for being a mermaid and fish that are friends living together. The two swim away from the commotion before the sea creatures prepare to attack. Meanwhile, Ariel and Flounder recall when they first met each other and became friends. After recalling when they first met, Ariel, with her determination renewed, goes out to prevent her friends from destroying each other as Ariel and Flounder plan to stop the prejudice-turned sea creatures from hating each other.
Meanwhile back at Atlantica, Flounder yells at the angry creatures that Ariel is telling the truth, but they refuse to listen and continue following the Manta's orders. Ariel then reveals that the Manta was right; the sea creatures are all different, but instead of hating each other, they should celebrate what makes each and everyone of them special by singing a song that brings all the sea creatures together. By doing so, their lives can be in harmony. Ariel coaxes the Atlanticans into reforming their friendships as they get along together. After misunderstanding, the Atlanticans remember what the Manta told them as Ariel tells them to ignore whatever the Manta tells them.
The Manta plans to finally turn Ariel against her friends. He suddenly hears music and realizes that the sea creatures are no longer hating each other but living together in harmony. Ariel suddenly arrives with all of the other creatures living and singing together in harmony. The Manta is powerless to the joy and fun that all of Atlantica has regained. Humiliated, he leaves in anger and fear, and Ariel congratulates her friends, saving the kingdom from the Manta. 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."