Those who are resistant to death are called "Demi-humans". That day, Kei Nagai, a high-school student, should have died in a traffic accident, but he comes back to life shortly after. In other words, Kei is a demi-human. Since then, Kei's world changes dramatically. Terrified and without knowing what is going on, Kei is saved by his friend, Kai. Together, they flee deep into a deserted mountain. Later, a group of demi-humans who are hostile against humans contact Kai… Who is he fighting against? Who should he side with?