The enigmatic immortal humans known as "Ajin" initially appeared in Africa 17 years ago. They were deemed a threat to humanity upon their discovery, as they could use their abilities for evil and were unkillable. Since then, whenever an Ajin is discovered in society, they must be instantly apprehended and brought into captivity. Kei Nagai, a high school student working hard to become a doctor, knows very little about Ajin, having only seen them in the news on occasion. Although students are taught that these creatures are not human, Kei does not pay attention in class. As a result, his dangerously limited understanding of the issue becomes utterly irrelevant once he lives.