Manatsu meets the "Legendary Pretty Cure" in her dream. According to Laura, the Legendary Pretty Cure saved the world of mermaids and humans a long time ago. Meanwhile, multiple Chou Zettai Yaraneeda are born from water, and appear throughout the town.

Manatsu and her friends head back to Grand Ocean to save the real Mermaid Queen. But then they discover a mysterious room filled with memories of mermaids who once interacted with humans. While there, Laura discovers a secret...