Five years have passed since Kotaro Minami defeated the malevolent creation cult Gorgom. Trying to leave his life as a cyborg superhero behind, Kotaro has found peace working as a helicopter pilot for the Sahara family, who have taken him into their home. His quiet life is shattered with the arrival of the Crisis Empire, a conquering alien race that sets its sights on Earth. Recognizing Kotaro as Kamen Rider Black, a potential threat to their invasion, the Empire captures him. When he refuses to join their cause, they shatter his transformation belt, the King Stone, and cast him into the cold emptiness of space to die.

Instead of perishing, the intense radiation of the sun revives Kotaro. The energy transforms his broken King Stone, evolving his body into a new, more powerful form: Kamen Rider Black RX, the Child of the Sun. His loyal companion, the motorcycle Battle Hopper, is also resurrected and upgraded into the living machine Acrobatter. Now reborn with new abilities, Kotaro wages a one-man war against the Crisis Empire, determined to protect Earth and his new family.

Kamen Rider Black RX is not merely an upgraded version of his former self; he pioneers the ability to change his structure to adapt to any battlefield. When pure power is not enough, he transforms into Roborider, the Prince of Sadness. This heavy, mechanized form sacrifices speed for immense physical strength and devastating armor, making him impervious to explosions and armed with the pistol-like Vortech Shooter. When facing agile or amorphous foes, he becomes Biorider, the Prince of Anger. This sleek, silver-and-blue form can move at incredible speeds and even liquefy his body into a fluid, invincible gel state to slip through traps or reform for an attack, wielding the flexible Bio Blade.

Kotaro does not fight alone. He is aided by Reiko Shiratori, a fearless photographer who becomes a close ally, and Joe the Haze, a mysterious cyborg originally sent by the Crisis Empire to destroy RX. After being defeated, Joe regains his lost memories and becomes one of Kotaro’s most trusted friends. He also finds support from the bickering yet loving Sahara family, including the energetic son Shigeru and young daughter Hitomi. Later in the series, he is joined by Kyoko Matoba, a young woman with powerful ESP abilities who wields a bow in battle.

As the series progresses, Kotaro faces not only the generals of the Crisis Empire—the scheming Maribaron, the naval commander Bosgun, and the robotic Gatezone—but also a ghost from his past. His adoptive brother, Nobuhiko Akizuki, who was transformed into the tragic warrior Shadow Moon during the events of Kamen Rider Black, is resurrected. Consumed by his programming and jealousy, Shadow Moon returns as a primary rival, forcing Kotaro to fight the brother he once tried to save. Their conflict reaches a climax when RX impales Shadow Moon with his Revolcane sword, shattering the dark King Stone and finally allowing Nobuhiko to die with his humanity restored.

In the final arc, the Crisis Emperor himself takes notice of RX’s repeated interference. After Kotaro is gravely wounded and his transformation watch is destroyed, the original ten Kamen Riders from Kamen Rider 1 through ZX arrive from across time to aid him. With their combined power and the creation of a new, stronger form of the light-saber Revolcane, Kamen Rider Black RX leads the charge into the Crisis Empire’s homeworld, finally destroying the alien invaders and securing a peaceful tomorrow for Earth.