In 2071, the melting of the North Pole resulted in the submergence of land and the splitting of humanity into two distinct species. The first species reside in underwater cities while the second, dubbed Dugons, are those who have undergone genetic modification to flourish in the sea. These species are constantly at odds with each other, as the Dugons view the other as a source of nourishment.
To defend themselves from the Dugons, the inhabitants of Mari Ramelle have developed a state-of-the-art weapon - the Salacia mermaid robot. The robot, which requires two 13-year-old pilots to operate, serves as a small pilot cabin. Marin Yuuki and Ushio Mizushiro work together as pilots to protect their community.
As the conflict with the Dugons intensifies, Mari Ramelle continues to construct additional mermaid robots to aid in their battle.
