Due to global warming, much of the Earth is submerged in water in the future. Maia Mizuki, a 15-year-old orphan who has just graduated from middle school, applies to join the Ocean Agency, an elite paramilitary group that is part of the futuristic world government. Only the most intelligent, physically fit students are accepted, and Maia is determined to become one of them. Unfortunately, Maia fails her entrance exams and is subsequently evicted from her home, pickpocketed, abducted, and shot. With nowhere else to turn, she ends up joining Nereids, an unusual help-for-hire organization inspired by the Greek mythological Nereids, whose members include two women named Rena and Shizuka. Maia ends up working alongside these women, taking on everything from tracking down criminals to chasing after stray cats. As the series progresses, Maia becomes embroiled in a subplot involving a character named Daphne, and she begins to better understand the world she lives in, which is dominated by water. Most of the land has been swallowed up by the rising oceans, and the world consists of vast oceans, a handful of islands, and floating cities. The ocean is both beautiful and treacherous, and Maia must learn to navigate its dangers if she hopes to survive.