Manatsu Natsuumi is a first-year middle school student who moved to Aozora City from Minamino Island to see her mother. Upon her arrival on the mainland, she begins her new life there and her first semester in middle school. But one day, she met with a strange person named Laura, a mermaid who had originated in a mystical place called Grand Ocean, the land of Mermaids. She has been searching for the legendary warriors of the Grand Ocean who can stand up against the Witch of Delays, an entity from the bottom of the ocean that conquered her nation and has sucked all the Motivation Power out of the people in the kingdom. Laura is only searching for the legendary warriors due to the request from the Queen of Mermaids and just for her own selfish desire to be queen one day. But as one of the witch's minions started to attack Aozora City, Manatsu suddenly is blessed with the powers of the sea that allows her to become Cure Summer, one of the legendary warriors foretold in the legends of the Grand Ocean.
Manatsu is later joined by Sango Suzumura (Cure Coral), Minori Ichinose (Cure Papaya), Asuka Takizawa (Cure Flamingo) and Laura (Cure La Mer) to form the Tropical-Rouge Pretty Cure Team to fend off against the forces of the Witch of Delays and restore the Grand Ocean to its former splendor.
