Kotaro Minami and Nobuhiko Akizuki are stepbrothers who were both born on the day of a solar eclipse, a rare event that marks them as destined candidates for a dark prophecy. Orphaned at a young age, Kotaro was raised by Nobuhiko’s father, Soichiro Akizuki, and the two grew up as close as true siblings, unaware of the sinister fate awaiting them. On their nineteenth birthday, they are kidnapped by Gorgom, a powerful and ancient secret cult that blends mysticism with advanced cybernetics. Gorgom’s goal is to select its next ruler, the Creation King, through a ritual combat between two enhanced warriors known as the Century Kings. The brothers are surgically modified into grasshopper-like cyborgs, each implanted with a mystical Kingstone: Kotaro receives the Stone of the Sun, designating him as Black Sun, while Nobuhiko receives the Stone of the Moon, becoming Shadow Moon.

Just before the final stage of brainwashing that would erase his will, Kotaro is rescued by Soichiro, who has a last-minute change of heart. Soichiro reveals Gorgom’s secrets but is killed by the cult while helping Kotaro escape. Nobuhiko, however, remains in Gorgom’s grasp and is placed into a comatose state to complete his transformation. Using the power of his Kingstone, Kotaro learns to transform into a powerful armored hero. Rejecting the name Black Sun, he renames himself Kamen Rider Black and vows to use Gorgom’s own gift to fight against them, to avenge his foster father, and to save his stepbrother from their clutches. From his base in Tokyo, he battles the monstrous mutants sent by Gorgom’s three High Priests—Darom, Baraom, and Bishum—who rule the cult in the name of the disembodied Creation King. Kotaro is aided by two unique motorcycles: the Battle Hopper, a sentient grasshopper-like machine created by Gorgom for the Century King, and the Road Sector, a high-speed vehicle.

For much of the series, Kamen Rider Black fights a lonely war against the cult’s monster-of-the-week schemes while desperately searching for a way to rescue Nobuhiko. The stakes rise dramatically when the ancient warrior Bilgenia is unleashed. A powerful fighter born during a solar eclipse but denied a Kingstone, Bilgenia acts as a wild card, targeting Kotaro’s friends and even stealing the legendary Satan Saber for his own ambitions. However, Bilgenia is ultimately dispatched with ease by the long-dormant Nobuhiko when he finally awakens.

Nobuhiko fully emerges as the dreaded Shadow Moon, now a powerful and ruthless warrior completely loyal to Gorgom’s cause and determined to fulfill the prophecy by killing Kotaro and becoming the new Creation King. Unlike the heroic but tragic Kotaro, Shadow Moon embraces his dark destiny and wields the Satan Saber with deadly skill. The brothers are forced into a heartbreaking series of duels, culminating in a final battle where Kotaro, his Kingstone shattered and his body broken, is seemingly killed. Resurrected by the life essence of a repentant whale monster and the prayers of his friends, a revitalized Kamen Rider Black faces Shadow Moon one last time. In their final confrontation, Kotaro defeats his brother, who, in his dying moments, regains his original self and hands the Satan Saber to Kotaro. Using the weapon, Kamen Rider Black destroys the Creation King and the entire Gorgom headquarters, finally ending the cult’s evil reign. Though he triumphs, Kotaro is left to face an uncertain future, having lost his brother and his closest companions.