Due to the impacts of global warming, water will cover most of the Earth in the future. Maia Mizuki, an orphaned 15-year-old, has just graduated from middle school and has already applied for a job with the elite paramilitary Ocean Agency, which is part of the futuristic world government. Only the brightest, most brilliant, and physically fit pupils are admitted. Maia, the heroine of the story, is about to become one of the few. Her idyllic existence, however, rapidly unravels. Maia's admission examinations are suddenly failed, much to her dismay. To make matters worse, she is subsequently evicted from her home, pickpocketed, kidnapped, held captive, and shot. She is "rescued" by two ladies (Rena and Shizuka) who work for an unconventional help-for-hire firm called Nereids (inspired by the Greek word for "saved").