Voyage Into Space (1970)

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.

Unfortunately the movie Voyage Into Space is not yet available on HBO Max.