Facing destitution after being ousted from his orphanage, Atsushi Nakajima's fortunes take a turn when he rescues a mysterious man, Osamu Dazai, from a watery grave. Despite the skepticism of Doppo Kunikida, Dazai reveals their affiliation with the Armed Detective Agency, a covert group handling cases beyond conventional law enforcement. Tasked with apprehending a rampaging supernatural tiger, Atsushi reluctantly joins forces with them. Subsequent success leads to an offer to join the agency, just as the dangerous Port Mafia sets their sights on them. As Atsushi navigates the dangerous waters of gang conflict, he must harness his latent powers to safeguard himself, his comrades, and the city at large.