The story begins with two young men, Kotaro Minami and Nobuhiko Akizuki, who were born on the same day during a solar eclipse and raised as brothers in Japan. On their nineteenth birthday, they are kidnapped by Gorgom, a sinister and ancient cult-like organization that secretly plans to rule the world. Gorgom’s leaders, the Three High Priests Darom, Baraom, and Bishium, select the pair to undergo a horrific cyborg modification surgery because of their birth under the eclipse, a sign that marks them as the chosen candidates to become the next Creation King, the absolute ruler of Gorgom. Kotaro is implanted with the Kingstone of the Sun, transforming him into the warrior designated as Black Sun, while Nobuhiko receives the Kingstone of the Moon, becoming Shadow Moon. The process is designed to strip them of their memories and will, turning them into loyal soldiers who will eventually fight each other to the death, with the survivor ascending to the throne of the Creation King.

During the final stage of the brainwashing, Nobuhiko’s father, Professor Akizuki, who is also a high-ranking Gorgom scientist, has a change of heart and helps Kotaro escape. The professor is mortally wounded in the process, but before dying, he reveals the truth to Kotaro and urges him to use his new powers to stop the evil cult. Kotaro flees Gorgom’s headquarters and discovers that he can transform into a powerful grasshopper-like cyborg. Rejecting the title of Black Sun, he renames himself Kamen Rider Black and dedicates his life to avenging his foster father and rescuing his stepbrother Nobuhiko. Early in his fight, he is aided by the sentient motorcycle Battle Hopper, a living machine that becomes his trusted partner, and later obtains the super-powered armored buggy Road Sector. He also receives occasional assistance from Kyoko Akizuki, Nobuhiko’s younger sister, and her close friend Katsumi Kida, who provide emotional support and investigate Gorgom’s activities.

For much of the series, Kotaro battles against Gorgom’s seemingly endless supply of mutant monsters, known as Kaijin, who are created to carry out schemes that range from brainwashing the public and manipulating nature to recruiting new members and gathering energy for the Creation King’s resurrection. He confronts the organization’s hierarchy, which includes not only the three High Priests but also powerful enforcers such as the sword-wielding Bilgenia and the human scientist Kuromatsu Hideomi, who defects from Gorgom after realizing the error of his ways. Despite Kotaro’s efforts, the shadow of his brother looms over every battle. Nobuhiko remains in a coma for many episodes, kept alive by Gorgom’s technology, and the cult is determined to awaken him as the perfect, loyal Shadow Moon.

The central narrative arc shifts dramatically when Nobuhiko finally awakens as Shadow Moon. He is more powerful than Black and is armed with the legendary Satan Sabre. Unlike Kotaro, who clings to his humanity, Shadow Moon embraces his destiny as the Century King and views his former brother as an obstacle to be eliminated. This transformation turns the story into a tragic conflict between two men who grew up as family but are now fated to fight for the future of the world. Kotaro refuses to give up on his brother, even as Shadow Moon ruthlessly attacks him and his friends. In the final episodes, the Three High Priests are defeated one by one, and Kotaro confronts the Creation King himself, a monstrous and pitiful creature kept alive within a massive life-support system. The series concludes with a final, brutal battle between Kamen Rider Black and Shadow Moon. After a long struggle, Kotaro is victorious and destroys Gorgom for good, but Nobuhiko is presumed dead, leaving Kotaro to walk away from his past life, having saved the world at the cost of losing the only brother he ever knew.