The story follows a high school student named Nagai Kei, who died in a car accident and was brought back to life because he is an Ajin. Ajin are beings that regenerate their bodies after death, as if they are invulnerable to harm. Fearing capture and being used as a test subject, our hero escapes and eventually becomes part of the ongoing conflict between the Ajin and humans, facing the challenge of choosing to help one side or the other.