Kei Nagai, a high school student, discovers he is an Ajin, a rare type of immortal human, after surviving a fatal accident. Ajin are feared and hunted by society, often subjected to inhumane experiments by governments and private organizations. Kei goes on the run, pursued by relentless forces seeking to capture him. Along the way, he encounters other Ajin, including the enigmatic and ruthless Satō, who leads a group of Ajin intent on waging war against humanity. Satō’s methods are extreme, involving acts of terrorism and violence to expose the cruelty of the system that oppresses Ajin.

Kei struggles with his newfound identity and the moral implications of his immortality. He is initially reluctant to embrace his abilities, which include summoning an invisible, ghostly entity known as an IBM (Invisible Black Matter). As the conflict escalates, Kei is forced to confront the harsh realities of the world and the choices he must make to survive. His journey is marked by alliances with both humans and Ajin, including Kō Nakano, a former classmate who becomes his ally, and Izumi Shimomura, a government agent with her own complex motivations.

The narrative explores themes of survival, morality, and the consequences of immortality in a world that fears and ostracizes the Ajin. Kei’s evolution from a passive, self-preserving individual to a more proactive and strategic thinker is central to the story. The series balances intense action sequences with psychological depth, examining the ethical dilemmas faced by both the Ajin and their pursuers. The tension between Kei and Satō serves as a driving force, culminating in a high-stakes battle that challenges the boundaries of humanity and immortality.