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?
Peace has returned to the underwater world, and the princesses head back to school. Purple is shocked to find out that Leo has been keeping in touch with the other princesses during the break— but not with her. Confused and hurt by Leo's cold attitude, Purple sees that he's now spending more time with Gray, a second grader at school. Feeling left out, she leaves her princess dictionary behind and runs off to the beach alone.
The other princesses still concerns about why Indigo never shows up until they learn that Indigo is at the Abyssal Kingdom learning that the blobs are still alive until Pale appears to make the other princesses break up with each other.
Purple and Indigo, who are growing increasingly misunderstood because they don't trust each other, gradually come to know each other's true feelings as they go to the secret temple...
Yellow has a timid personality and finds it difficult to say no. She tries to avoid awkward situations by creating a clone of herself...
Purple refuses to go home because her test scores are bad! Then she gets attacked by Black Diamond's subordinates..
Hearing that they'll receive volunteer points, Gray and Green recommend decorating the hallway to the princesses. Blue and Red decorate it however they want, placing a red carpet on one side and exercise equipment on the other. Meanwhile, Black Diamond creates a new Black Curse, her most powerful subordinate, Dark. After obtaining Green's Bubble Gem, Dark lifts Blue Dot School into the sky and turns it into a maze. The school, already complicated by the red carpet and exercise equipment in the hallway, becomes even more confusing, and Indigo, who had been suspicious of Gray from the beginning, confronts Gray.
The princesses must escape the school that is as tangled as a maze. Meanwhile, Indigo, who suspects Gray, ends up fighting with Purple...
Mr. Snivel, who has been suspicious of Purple due to the strange things that have been happening at school, secretly follows Purple and ends up witnessing the princesses' true identity!
The princesses participate in the parade. However, Black Diamond, who denies the legend of mermaids and humans, attacks them...
Purple heads to the Jellyfish Kingdom to find a scroll about the ancient kingdom. She plans to trick King Blue King.
