"Marine Snow no Densetsu" is a science fiction anime set in a future where humanity has colonized the ocean floor. The story centers on a young boy named Toto, who lives in an underwater city called Marinepolis. Toto's life takes a dramatic turn when he encounters a mysterious girl named Marine, who possesses the ability to communicate with marine life. Marine is revealed to be a member of an ancient underwater civilization known as the "Blue People," who have long been hidden from human society.

The narrative unfolds as Toto and Marine embark on a journey to uncover the truth about the Blue People and their connection to the ocean's ecosystem. Along the way, they face opposition from the Surface Dwellers, a group of humans who seek to exploit the ocean's resources without regard for its delicate balance. The Surface Dwellers view the Blue People as a threat to their ambitions and attempt to capture Marine to harness her abilities for their own gain.

As Toto and Marine delve deeper into their quest, they discover that the ocean is home to a powerful entity known as the "Marine Snow," a phenomenon that sustains the underwater world but is threatened by human interference. The duo must navigate treacherous waters, both literal and metaphorical, to protect the Marine Snow and prevent the destruction of their home. Their journey is fraught with challenges, including encounters with hostile sea creatures, the machinations of the Surface Dwellers, and the internal conflicts within Marinepolis itself.

Key characters include Toto, whose curiosity and bravery drive the narrative; Marine, whose connection to the ocean and its inhabitants is central to the plot; and various members of the Surface Dwellers and Marinepolis, whose actions and motivations add layers of complexity to the story. The series explores themes of environmentalism, the clash between technological advancement and natural preservation, and the importance of understanding and respecting the balance of ecosystems.

The plot builds towards a climactic confrontation between the forces seeking to exploit the ocean and those striving to protect it. The resolution hinges on the choices made by Toto and Marine, as well as the broader implications for the future of both human and underwater civilizations. "Marine Snow no Densetsu" presents a compelling narrative that intertwines adventure, mystery, and ecological awareness, set against the backdrop of a richly imagined underwater world.