"Aqua Age" is a science fiction manga set in a future where Earth's surface has become uninhabitable due to environmental collapse, forcing humanity to live in underwater cities. The story centers on Kaito, a young engineer who works on maintaining the fragile infrastructure of Aqua Metropolis, one of the largest underwater settlements. Kaito's life takes a dramatic turn when he discovers a hidden archive containing blueprints for an ancient surface-dwelling civilization's technology, which could potentially restore Earth's surface to a habitable state.

As Kaito delves deeper into the archive, he uncovers a conspiracy involving the ruling council of Aqua Metropolis, who have suppressed this technology to maintain control over the population. Kaito is joined by a group of allies, including Mira, a marine biologist with a deep understanding of oceanic ecosystems, and Ren, a former soldier disillusioned with the council's authoritarian rule. Together, they form a resistance movement to challenge the council and bring the suppressed technology to light.

The narrative explores themes of environmental stewardship, political corruption, and the ethical implications of technological advancement. Kaito and his allies face numerous challenges, including sabotage from the council, the dangers of the deep sea, and internal conflicts within their group. The story builds toward a climactic confrontation as the resistance attempts to deploy the ancient technology, risking everything to give humanity a chance to return to the surface.

The characters are defined by their motivations and relationships, with Kaito's idealism contrasting with the pragmatic approaches of Mira and Ren. The council's leaders, particularly the enigmatic Chairman Ryusuke, serve as formidable antagonists, driven by a belief that humanity's survival depends on maintaining the status quo. The manga balances action, intrigue, and emotional depth, creating a compelling narrative that examines the consequences of humanity's relationship with the environment and technology.