After alien lifeforms known as "Trenchwalkers" land in Antarctica, they transform the planet, causing sea levels to rise by 70 meters and submerging cities like Tokyo. Furthermore, they begin converting Earth’s water into a divine substance, "Nectar," which proves fatal to those whose bodies cannot adapt to it. A handful of girls who can withstand the Nectar are chosen to board a submarine, equip special swimsuits, and head to the battlefield to reclaim our planet. Upon receiving the dying message of her older sister, Todoroki Hiro goes in search for the strongest marine soldier Tsunoba Kiriri, and despite her heart being full of uncertainties, she chooses to enroll in the Masakado Girls' Academy Submarine Fleet. However, will it actually be smooth sailing for her?! Girls in swimsuits, water guns, and high-stakes combat: the ultimate splash-action battle for Earth is here!