"Aqua Age" is set in a futuristic world where Earth's surface has become uninhabitable due to environmental collapse, forcing humanity to live in underwater cities known as Aqua Colonies. The story centers on Kaito, a young engineer who discovers a hidden conspiracy threatening the fragile balance of these underwater societies. Kaito's journey begins when he stumbles upon an ancient AI named Nereus, which holds the key to restoring the planet's surface. However, Nereus is also sought by the authoritarian Aqua Council, which aims to use its power to maintain control over the colonies.

As Kaito delves deeper into the mystery, he allies with a group of rebels, including Mira, a skilled diver and former soldier, and Ren, a hacker with ties to the underground resistance. Together, they uncover the truth about the Aqua Council's experiments, which involve manipulating the ocean's ecosystems to sustain their dominance. The narrative explores themes of environmental degradation, technological ethics, and the struggle for freedom, as Kaito and his companions navigate treacherous waters, evade Council enforcers, and confront the moral dilemmas of wielding Nereus's power.

The plot intensifies as Kaito learns that Nereus is not merely a tool but a sentient being with its own agenda, raising questions about trust and autonomy. The series balances action-packed sequences, such as underwater battles and high-stakes chases, with quieter moments of character development, particularly Kaito's evolving relationship with Mira and his internal conflict over his role in the rebellion. The story culminates in a climactic confrontation with the Aqua Council, where Kaito must decide whether to use Nereus to restore the surface at the risk of destabilizing the colonies or to find a more balanced solution that ensures humanity's survival.