DR. KOMYOGI is an expert in robotics and has a wealthy sponsor, HERBERT GILL. Gill however, is a horrible man who forces Dr. Komyogi to develop numerous robots which he plans to use to take over the world. Dr. Komyogi realizes Gill's dark ambition and secretly develops a high-performance robot called KIKAIDER. He leaves Kikaider with his daughter MITSUKO and then disappears.
Finally, Gill makes his move. He unleashes a robot army named DARK to dominate and control human society. Mitsuko, reading a message left for her by her father Dr. Komyogi, persuades Kikaider to fight off Gill's terrible robots. On that day, Kikaider's fierce battles with DARK begin!
Kikaider, however, is a robot imprinted with a 'conscience circuit,' so that he can act more like a human. As a result of this circuitry, Kikaider suffers from the agony of being neither truly human nor truly robot. As he spends time with Mitsuko, he gradually becomes conscious of what it really means to be 'human'--
Kikaider is supposed to protect humans. However, the evil Professor Gill who funded the scientist sends Kikaider's brother figures, robot army corps as hatchet men in order to destroy Kikaider who is in the professor's way. This means to Kikaider having to kill his own brothers in order to protect humans.