"Deep Blue Song" is a science fiction anime series set in a future where humanity has colonized the ocean floor to escape the environmental collapse of the surface world. The story centers on Aoi, a young marine biologist who specializes in studying the mysterious bioluminescent creatures that inhabit the deep sea. Aoi’s life takes a dramatic turn when she discovers an ancient, sentient underwater city that holds the key to humanity’s survival. The city, known as "Atlantica," is powered by a unique energy source derived from the bioluminescent organisms Aoi has been researching.

As Aoi delves deeper into Atlantica’s secrets, she uncovers a hidden conflict between the surface-dwelling humans and the ocean’s indigenous inhabitants, a highly intelligent species known as the Nereids. The Nereids, who have long been misunderstood and exploited by humans, view Atlantica as their ancestral home and are determined to protect it from further human interference. Aoi becomes a mediator between the two factions, striving to find a balance between humanity’s need for survival and the Nereids’ right to preserve their culture and environment.

The narrative explores themes of environmentalism, coexistence, and the consequences of technological advancement. Aoi is joined by a diverse cast of characters, including Kaito, a former surface engineer who becomes her ally in understanding Atlantica’s technology, and Lumi, a Nereid who forms a bond with Aoi and helps bridge the gap between their species. The series also features antagonists such as Dr. Ren, a ruthless scientist who seeks to exploit Atlantica’s energy for personal gain, and the Surface Council, a governing body that prioritizes human survival over ethical considerations.

The plot unfolds through a series of escalating conflicts, as Aoi and her allies work to prevent a catastrophic war between humans and Nereids. The story is driven by Aoi’s growth as she grapples with moral dilemmas, her evolving understanding of the ocean’s ecosystem, and her role as a bridge between two worlds. The series concludes with a resolution that emphasizes the importance of mutual respect and cooperation, leaving room for future exploration of the world and its characters.