The Final Showdown
The female chemist who has proposed a new use for volcanic energy, called the "Active Volcano Guiding System" at the energy chemists' conference, is named Irena Engel. The name means something to Akira; and he suspects that she could be somehow related to Hans Engel. As feared, the conference venue is suddenly thrown into chaos by an ultrasonic wave beam, and Irena is kidnapped and her research confiscated. The culprit is undoubtedly Hans. He has been planning to threaten the world by converting the new volcano energy use system, developed by his sister Irena, into a missile. Irena grieves over the actions of her brother. But Hans is doggedly determined to take revenge on Dr. Asagumo, who developed Big X, as he believes that his father was killed by the doctor. Hans challenges Akira to a final showdown, expecting to settle old scores at last. Akira, however, tries to persuade Hans not to fight a useless battle. Irena also begs him to return to being the kind brother that he used to be. Source: tezukaosamu.net
EP 59 - The Final Showdown
Big X
Invited to Nazi Germany during World War II, Dr. Asagumo is asked by Hitler to collaborate with him on the research of the new weapon "Big X". Concerned about the possible effects of the completion of Big X, Dr. Asagumo intentionally delays the progress of the research, conspiring with his co-researcher, the devious Dr. Engel. Immediately before Germany is defeated by the Allies, Dr. Asagumo implants a card inscribed with the secret of Big X in his son, Shigeru, and is then shot to death by the German army. Twenty years later, the card is discovered in the body of Shigeru, who is then living in Tokyo. Soon, an organization claiming alliance with the Nazis appears, steals the card, and completes the Big X project. Dr. Engel's grandson has joined the Nazi Alliance. The completed Big X is then revealed to be a drug that can expand the human body without limitation. Recovering Big X from the enemy, Shigeru's son Akira fearlessly challenges the Nazi Alliance and Hans Engel, who are plotting to conquer the world.
Note: Most of the series is lost media. Only episodes 1, 11, 12 & 40-59 are known to still exist.