"Tokyo Yamimushi" is a manga series set in a dystopian version of Tokyo, where society has collapsed due to a mysterious phenomenon known as the "Night Bug" infestation. The Night Bugs are parasitic creatures that emerge at night, preying on humans and turning them into mindless, violent beings. The story centers around a group of survivors navigating this perilous world, struggling to maintain their humanity while facing constant threats from both the Night Bugs and other desperate humans.

The protagonist, a young man named Shou, is a former high school student who becomes a reluctant leader among the survivors. Shou is pragmatic and resourceful, often making difficult decisions to ensure the group's survival. His moral compass is tested repeatedly as he confronts the harsh realities of their new world. Alongside Shou is Aya, a determined and compassionate girl who serves as a counterbalance to his pragmatism. Aya's unwavering hope and empathy often clash with Shou's more cynical outlook, creating tension within the group.

Other key characters include Kengo, a former delinquent with a strong sense of loyalty, and Rika, a mysterious girl with a hidden connection to the Night Bugs. Kengo's brute strength and protective nature make him a valuable asset, while Rika's knowledge of the Night Bugs becomes crucial as the group uncovers the truth behind the infestation. The dynamics between these characters drive much of the narrative, as they grapple with trust, betrayal, and the struggle to retain their humanity in a world that increasingly devalues it.

The plot unfolds as the survivors search for a safe haven, encountering various factions of humans who have adapted to the new world in different ways. Some groups resort to violence and exploitation, while others attempt to rebuild a semblance of civilization. The Night Bugs themselves evolve over time, becoming more intelligent and organized, posing an ever-greater threat. The series delves into themes of survival, morality, and the fragility of human society, with the characters' choices often having far-reaching consequences.

As the story progresses, the origins of the Night Bugs are gradually revealed, tying into a larger conspiracy involving human experimentation and environmental collapse. The survivors must confront not only the external threats but also their own inner demons, as the line between humanity and monstrosity becomes increasingly blurred. The narrative builds toward a climactic confrontation with the forces behind the Night Bug outbreak, forcing the characters to make ultimate sacrifices in their quest for survival and redemption.