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The Shiranui clan have planned for 2000 years to conquer Japan, in retaliation for being banished from the country by the Yamato clan (who eventually become known as the Japanese people). Each member of the clan controls a gigantic robot warrior in order to overthrow the "Yamato Government," as they call it. In response to the threat, Japan's National Security Organization send agent Gentaro Shizuka (Shoji Ishibashi), best described as a singing cowboy, to stop their plans with the assistance of comical, mountain climbing-clad, Goro Kirishima (Mitsuo Hamada). In an unusual direction for such tokusatsu (visual effects) programs, it is not Gentaro but Goro who has the power to become the giant cybernetic hero Iron King by touching the medals on his Turning Hat, and shouting "Iron Shock!" when danger threatens.

However, often Iron King is unable to defeat the clan's giant robots without help from Gentaro, who wields a weapon called the Iron Belt that can become a slender sword or a whip capable of hurting giant monsters. Transforming into Iron King quickly dehydrates Goro and he can only remain as Iron King for a short period of time. Strangely, though Gentaro knows Iron King's time limit comes from dehydration he never manages to connect it with Goro's omnipresent thirst.

In the tenth episode the Shiranui Clan is destroyed but their place is taken by the Phantom Militia who also use giant robots to enact conquest of Japan, but rather than being humanoid in shape theirs take the form of kaiju. From the eighteenth episode to the end of the series Gentaro and Goro battle the alien Titanians who have various inhuman powers such as flight, mind control and the ability to enlarge their bodies to giant size. Upon doing so they are able to assume insect-like monster forms.