Despite their differences in position, three men—the youngest senior executive of the Port Mafia, Osamu Dazai; the lowest ranking member, Sakunosuke Oda; and the intelligence agent, Ango Sakaguchi—gather at the Lupin Bar at the end of the day to relax and take delight in the company of friends. However, one night, Ango disappears. A photograph taken at the bar is all that is left of the three together. Fast forward to the present, and Dazai is now a member of the Armed Detective Agency. The Guild, an American gifted organization, has entered the fray and is intent on taking the Agency's work permit. They must now divide their attention between the two groups, the Guild and the Port Mafia, who oppose their very existence. 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!