Deep in the sea where mermaids live, the Merqueen created magical Bubblegems to protect the undersea kingdoms from evil forces. Now it’s time to find her successor, and whichever princess can gather the Bubblegems will be the new queen. But the Bubblegems are scattered on the land, hidden in secret volcanos, so the seven mermaid princesses must emerge from the sea and go to human school while they search for the precious jewels. But someone else is after the Bubblegems – the wicked witch Black Dia, who is tired of her exile in the dark deep sea and wants to seize control of the sea world for herself. Will her black magic prove too powerful for the young mermaid princesses, or can they work together to collect all the Bubblegems and save the undersea kingdom?

The seven mermaid princesses, along with their new human friends including Leo's senior student named Gray, fight against Black Diamond, who has returned stronger than ever. Can Purple protect the ocean world, humans, and the land without her own Bubble Gem?