Despite their varying positions within the Port Mafia, Osamu Dazai-the youngest senior executive, Sakunosuke Oda-the lowest ranking member, and Ango Sakaguchi-the intelligence agent, regularly gather at the Lupin Bar to unwind and enjoy each other's company. However, Ango goes missing after one such night, and only a photograph of the trio remains. Fast forward to the present, and Dazai has joined the Armed Detective Agency. As the Guild, a gifted organization from America, strives to strip the Agency of its work permit, they must allocate their efforts between defending against the Guild's attacks and combating the Port Mafia, who seek their destruction.