The episode starts with Ariel and Flounder seeing off Triton, Sebastian and Urchin 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 and Flounder try to trap the Manta in a net, but he's too powerful and escapes easily. So they decide to use a different approach, and try to reason with the angry Atlanticans, and to help threm remember how wonderful their lives had all been when they were openly sharing their world. In particular Ariel tells them to believe in their hearts, and not to just blindly listen to what any outsider says about each other.