For close to a year, lovely teenage girl Arise Nakajima has lived in Yokohama alone. A month and three weeks after her nineteenth birthday, her boss Yukichi Fukuzawa calls her into his office, where he lets her know he had decided to select her to be the Armed Detective Agency member who would be transferred to the ranks of the deadly criminal organization the Port Mafia as the price of the deal he had made a whole month earlier with former doctor Ogai Mori, who had been killed and replaced as the criminal organization's leader by his assistant Osamu Dazai. Afterwards informed of the former doctor's death, Arise leaves the Agency's headquarters carrying a cardboard box she soon confirms had been sent to her boss by Osamu, who has been in love with her for seven whole months.