The first Ajin appeared 17 years ago and quickly became the most important discovery to mankind. While they appear to be human, Ajin are rare, immortal beings that are feared by the populace. Only 46 are known to exist in the world, and most try to hide their existence, for capture means being subjected to brutal experiments that never end. Kei Nagai is a boy who dreams of becoming a doctor to heal his sister one day, but after he dies - and revives - in a car accident, he learns the terrifying truth that he too is an Ajin. Now, Kei finds himself on the run from both human and Ajin monsters that wish to do him harm, while struggling with his new, unwanted identity.