After a tumultuous three-way power struggle, government official Ango Sakaguchi reflects on a past incident following the death of the Port Mafia's previous leader. Tasked with probing a mysterious explosion tied to the former boss's return, novice investigator Osamu Dazai is reluctantly paired with Chuuya Nakahara, the dynamic head of the rival Sheep clan. Together, they delve into the legend of Arahabaki, the fire deity central to cracking the case wide open. Meanwhile, in the present, the Armed Detective Agency enjoys a brief respite before facing a fresh wave of old and new adversaries, signaling another imminent clash on the horizon.