"NaQa-Den" is a manga series set in a dystopian future where humanity faces extinction due to a mysterious alien virus known as the NaQa. The virus infects both humans and machines, causing them to mutate into monstrous entities called "Denizens." The story centers on a group of survivors navigating this perilous world, struggling to uncover the origins of the virus and find a cure.

The protagonist, Kaito, is a former soldier who becomes immune to the NaQa virus after a near-fatal encounter with a Denizen. His immunity makes him a target for various factions vying for control in the chaos. Among these factions is the militaristic organization known as the Iron Fist, which seeks to weaponize the virus, and the underground resistance group, the Freeborn, which aims to protect the remaining uninfected population. Kaito is joined by a diverse cast of characters, including Aya, a scientist determined to find a cure, and Ryo, a rogue mercenary with a personal vendetta against the Iron Fist.

As the group travels across the desolate landscape, they uncover fragments of information about the virus's origins, which are tied to an ancient alien civilization that once thrived on Earth. The narrative explores themes of survival, morality, and the consequences of humanity's hubris in tampering with forces beyond its understanding. The story is punctuated by intense battles against Denizens and clashes with rival factions, as well as moments of introspection and character development.

The plot thickens as Kaito and his companions discover that the NaQa virus may not be entirely malevolent; it appears to have a symbiotic relationship with certain individuals, granting them enhanced abilities at the cost of their humanity. This revelation forces the group to confront difficult choices about the future of humanity and the nature of their own identities. The series builds toward a climactic confrontation with the Iron Fist and the enigmatic leader of the Denizens, who holds the key to the virus's true purpose.

"NaQa-Den" combines elements of science fiction, horror, and action, creating a tense and immersive narrative. The characters' struggles against external threats and internal conflicts drive the story forward, making it a compelling exploration of survival in a world on the brink of collapse.