Set in 1937 on the eve of World War II, the narrative centers on a clandestine spy training group called the "D Agency." Founded by Lieutenant Colonel Yuuki of the Imperial Japanese Army, the organization recruits individuals beyond standard military academy graduates and personnel, driven by his ideals to train them in the arts of manipulation. These operatives form a specialized team for conducting missions. One such antihero, Jirou Gamou, undertakes a grueling assignment to locate secret documents known as the "Black Notes," while contending with threats from both inside and outside his own ranks.