"Ajin" is a dark fantasy anime series based on the manga by Gamon Sakurai. The story is set in a world where a small percentage of the population, known as Ajin, are immortal beings with regenerative abilities. These individuals are feared and hunted by governments and organizations due to their perceived threat to humanity. The narrative centers on Kei Nagai, a high school student who discovers he is an Ajin after surviving a fatal accident. His newfound immortality forces him into a life on the run, pursued by authorities and shadowy groups seeking to exploit Ajin for experimentation.

Kei's journey intertwines with other Ajin, including Sato, a charismatic yet ruthless figure who leads a militant faction of Ajin. Sato's goal is to incite chaos and rebellion against the human world, using his strategic brilliance and combat skills to challenge the government. Kei, initially reluctant to embrace his identity, is drawn into the conflict as he grapples with his morality and the ethical implications of his immortality. Alongside Kei is his loyal friend Kaito, who remains by his side despite the dangers, and Tosaki, a government agent with ambiguous motives who seeks to control or eliminate Ajin.

The series explores themes of survival, identity, and the moral complexities of immortality. It delves into the psychological and physical toll of being hunted, as well as the ethical dilemmas faced by both Ajin and humans. The plot is driven by intense action sequences, strategic battles, and the evolving dynamics between the characters. Kei's transformation from a passive individual to a more assertive and calculating figure is a key element of the story, as he navigates the treacherous landscape of being an Ajin in a world that fears and despises him. The narrative builds toward a climactic confrontation between Kei, Sato, and the forces seeking to control or destroy the Ajin, leaving lasting consequences for all involved.