This story takes place in the not-too-distant future, where humanity has been subjected to an attack by virus-carrying parasites called Gasteria. As a result, being expelled from their native homes, the people exist only in a small territory where, having already lost all hope, they live side by side with death. In this dark world lives Rentaro, a young man whose home is near Tokyo and who is a member of the "Civil Security"—an organization that fights those very Gasteria parasites. Together with his partner Enju, they carry out the most risky missions. Carrying out one mission more dangerous than another, working day after day, one day they receive a special assignment from the government to prevent the destruction of Tokyo. And thus begins the narrative about one of the most significant missions for protecting humanity.