Manatsu Natsumi, a first-year junior high school student, was born and raised on a small island and has been attending junior high school in the city of Aozora since spring. On the day she moves to Aozora City, Manatsu has a shocking encounter with Laura, a mermaid who is looking for a legendary warrior, Precure.

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.