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 agents form a specialized unit for conducting operations. One such antihero, Jirou Gamou, undertakes a harrowing mission to recover secret documents known as the "Black Notes," while fighting threats from both within and outside his ranks.