In a future where the Earth is mostly submerged under water due to global warming, Maia Mizuki, a 15-year-old orphan, aims to join the prestigious Ocean Agency, a part of the world government. Despite her high hopes, Maia's dreams shatter when she fails the entrance exams and faces a series of unfortunate events, including eviction, theft, abduction, and injury. Rescued by two unconventional women named Rena and Shizuka from the Nereids organization, Maia reluctantly becomes a member of their team. Together, they take on various tasks from apprehending criminals to chasing stray animals, leading to unexpected outcomes. Joined by Gloria and Yu, the group navigates a world dominated by water, sparse land, floating cities, and a subplot involving 'Daphne,' which proves crucial to Maia's journey. The title 'Brilliant Blue' symbolizes this aquatic world's expanse and the challenges it presents to Maia and her comrades.