"Big X" is a science fiction anime series that revolves around the conflict between two powerful organizations, the secretive Nazi group known as the Schwarze Hölle and the Allied forces' scientific research organization, the X Agency. The story begins during World War II, where Dr. Asagumo, a brilliant scientist working for the X Agency, develops a revolutionary serum called "Big X" that grants superhuman abilities. However, the Schwarze Hölle seeks to steal this technology to further their own nefarious goals. In a desperate attempt to protect the serum, Dr. Asagumo injects his son, Akira, with Big X, transforming him into a superhuman warrior capable of combating the Schwarze Hölle's forces.

Years later, the Schwarze Hölle resurfaces, now led by the enigmatic Dr. Engel, who continues to pursue the Big X technology. Akira, now an adult, is drawn back into the conflict as he inherits his father's legacy and becomes the central figure in the battle against the Schwarze Hölle. Alongside allies such as Shigeru, a skilled pilot, and Maki, a resourceful agent, Akira confronts the organization's advanced weaponry, including giant robots and genetically enhanced soldiers. The series explores themes of scientific responsibility, the consequences of war, and the struggle between good and evil, as Akira and his team work to prevent the Schwarze Hölle from achieving global domination.

The narrative is structured around episodic missions, with each episode featuring Akira and his allies facing new threats from the Schwarze Hölle. The series combines elements of espionage, action, and science fiction, with a focus on the moral dilemmas posed by the misuse of advanced technology. Akira's journey is one of self-discovery and resilience, as he grapples with the burden of his powers and the legacy of his father's work. The Schwarze Hölle serves as a persistent and formidable antagonist, with Dr. Engel embodying the dangers of unchecked ambition and scientific exploitation. Through its dynamic storytelling and exploration of ethical questions, "Big X" presents a compelling tale of heroism and the enduring fight against tyranny.