Earth is invaded by an interstellar terrorist group, Big Fire (the Gargoyle Gang in the American version), led by Emperor Guillotine who spends most of his time in a multicolored space ship hidden at the bottom of Earth's ocean, from which he issues his orders. Big Fire is capturing scientists to create an army of monsters to conquer Earth. A boy named Daisaku Kusama (Johnny Sokko in the American version) and a young Unicorn agent named Jūrō Minami (Jerry Mano in the American version) are shipwrecked on an island after being attacked by a sea monster and subsequently captured by Big Fire. They flee to where a Pharaoh-like giant robot is being built by captive scientist Lucius Guardian, who gives Daisaku and Jūrō its control device. Guardian helps them escape before he is shot to death; before he dies, he triggers an atomic bomb which destroys the base. The radiation activates the robot, which now obeys only Daisaku.

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