Black Dia is the main antagonist and former ruler of the Seven Seas. Her goal is gain control of both the land and the sea.

Rainbow Bubblegem recounts the magical adventures of seven mermaid princesses who must go to school on land to search for enchanted jewels to save their undersea realm from an evil witch.

When the bubble gems scatter, exhausting their magical powers, peace returns to the sea, and Purple wants to go back to school on land, but the Purple King is stubbornly opposed. After some serious convincing, Purple announces her intention to run for queen, and is finally allowed to return to the surface. But what lies ahead for her?
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One day at the beginning of a new semester, Purple loses her magic wand and her only princess dictionary while trying to protect her friend Leo from the villainous Blobs, Black Diamond's subordinates. Then, White Gold, the guardian of the sea, senses the dark energy and gives her a new catcher and princess dictionary as gifts. With this, Purple creates new bubble gems for the other princesses and upgrades their magic, but she is unable to create a bubble gem with her own color.
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The seven mermaid princesses, along with their new human friends including Leo's senior student Gray, the seven mermaids battle the returning and more powerful Black Dia. Without her own bubble gem, can Purple protect the sea world, the humans, and the land world?