"Joker Game" is set in the years leading up to World War II and revolves around the D Agency, a covert intelligence organization established by Lieutenant Colonel Yuuki of the Imperial Japanese Army. The agency operates independently from military and government oversight, focusing on espionage and intelligence gathering. Its operatives, known as spies, are rigorously trained to prioritize logic and strategy over patriotism or loyalty, embodying the philosophy that "a spy must not die, must not kill, and must not be caught."

The series is episodic, with each episode typically focusing on a different D Agency operative and their mission. These missions span various international locations, including Europe, Asia, and the Middle East, reflecting the global scope of the agency's operations. The spies employ a range of tactics, from infiltration and disguise to psychological manipulation, to achieve their objectives. Their missions often involve thwarting foreign intelligence operations, uncovering secrets, or manipulating political outcomes to Japan's advantage.

Key characters include Lieutenant Colonel Yuuki, the enigmatic and calculating leader of the D Agency, who is both respected and feared by his subordinates. The operatives, such as Sakuma, Kaminaga, and Miyoshi, are highly skilled individuals with distinct personalities and backgrounds, though their true identities and motivations remain shrouded in mystery. Sakuma, a military officer initially skeptical of the D Agency's methods, serves as a recurring character who provides an outsider's perspective on the agency's unconventional approach to espionage.

The narrative explores themes of loyalty, deception, and the moral ambiguity of espionage. The spies often find themselves in morally complex situations, where the lines between right and wrong are blurred. The series also delves into the psychological toll of living a double life, as the operatives must constantly navigate a world of lies and subterfuge. The overarching storyline subtly builds tension as the world edges closer to war, with the D Agency's actions playing a crucial role in shaping the geopolitical landscape.

"Joker Game" combines historical context with fictional espionage, offering a grounded yet suspenseful portrayal of pre-war intelligence operations. Its episodic structure allows for a diverse range of stories, while the overarching narrative ties the missions together, highlighting the strategic importance of the D Agency in a world on the brink of conflict.