High school student Mayama possesses an unusual set of skills. Her father, now missing for six years, trained her from a young age in crisis management and survival tactics, preparing her for scenarios that no one else takes seriously. That changes when a group of classmates conducting a simple test of courage stumbles upon a horrifying scene at an abandoned building. They witness a boy from their class being consumed by other students, seemingly killed. Terrified, they do not report it, only for the supposedly dead boy to return to school the next day, physically present but with something clearly wrong about him.

This event marks the beginning of a slow, creeping apocalypse. The antagonist is a non-human entity referred to as "them" that has the ability to invade and take over human bodies. The story follows Mayama as she becomes the sole member of her friend group who understands the true nature of the threat. While her friends panic, Mayama comes alive in the chaos, using her analytical mind and survival instincts to navigate a world where no one can be trusted. The horror lies in the uncertainty and the slow erosion of normalcy as the entities infiltrate the school and spread through the community, turning everyday life into a relentless fight for survival.

The central narrative thrust follows Mayama as she takes a stand against the encroaching horror. In a key development, her friend Kumako is attacked by the entities and becomes a victim of possession. In an attempt to rescue her, Mayama pursues the threat into a love hotel, only to be attacked from behind. Simultaneously, another friend named Hattori is separated from the group after an argument, leaving her vulnerable as the entities extend their reach. The series chronicles the desperate, lonely fight of this young woman as her social circle collapses and her friends are systematically taken over, forcing her to rely on the survivalist skills she once thought were useless.