During World War II, Dr. Asagumo is summoned to Nazi Germany by Hitler to assist in developing a powerful new weapon called "Big X". Fearing the weapon's potential consequences, Dr. Asagumo hinders the research progress with the help of his cunning colleague, Dr. Engel. As the Allies close in on Germany, Dr. Asagumo embeds a secret card detailing Big X inside his son, Shigeru, before meeting his demise at the hands of the German army. Two decades later, the card is found in Shigeru, now residing in Tokyo. A group affiliated with the Nazis seizes the card, finishes the Big X project, and unveils it as a substance capable of endless human growth. Dr. Engel's grandson aligns with the Nazi faction. When Akira, Shigeru's son, retrieves Big X from the enemy, he bravely confronts the Nazi Alliance and their leader Hans Engel, who aim to dominate the world.