After the three-way organizational war concludes, Ango Sakaguchi, a government bureaucrat, reminisces about an incident that occurred many years ago. Following the death of the previous Port Mafia leader, Osamu Dazai, a fresh recruit, was tasked with investigating rumors surrounding a mysterious explosion that had destroyed a portion of the city years before. He was also asked to explore the deceased boss's alleged return. Due to factors beyond his control, Dazai was paired with Chuuya Nakahara, the talented but reckless leader of a rival faction known as the Sheep. Their mission was to uncover the case's truth and shed light on Arahabaki, the god of fire, whose involvement might lead Dazai to the solution. Meanwhile, in the present day, the Armed Detective Agency is resuming everyday activities. Unfortunately, old and new adversaries are gathering their forces and preparing for another confrontation, so their peaceful interval will be brief.