After losing interest in military affairs, Yukichi Fukuzawa becomes a freelance bodyguard using his lethal swordsmanship. Unfortunately, business is slow until he meets Ranpo Edogawa, a bold young boy. Their initial encounter is during a murder investigation, which leads to the formation of the Armed Detective Agency. Currently, Ranpo is searching for a dangerous criminal with the ability to commit the perfect crime. As he solves the case, he discovers an intricate plan to destroy the agency. Despite being warned of the trap, they pursue the perpetrators, but get blamed for the crime instead. Now, they are labeled as wanted terrorists and must seek help from their sworn enemies to clear their reputation.