Power users around the world are suddenly committing suicide one after another, in all cases after a strange fog appears at the scene. At a request from Ango Sakaguchi, the Armed Detective Agency head out to investigate Tatsuhiko Shibusawa, a power user who calls himself "collector," and a man suspected of being tied to the incident.
Nakajima Atsushi was kicked out of his orphanage, and now he has no place to go and no food. While he is standing by a river, on the brink of starvation, he rescues a man whimsically attempting suicide. That man is Dazai Osamu, and he and his partner Kunikida are members of a very special detective agency. They have supernatural powers and deal with cases that are too dangerous for the police or the military. They're tracking down a tiger that has appeared in the area recently, around the time Atsushi came to the area. The tiger seems to have a connection to Atsushi, and by the time the case is solved, it is clear that Atsushi's future will involve much more of Dazai and the rest of the detectives!