A young man named Ryo Murasame and his older sister Shizuka, a newspaper reporter, are investigating rumors of a UFO while flying over the Amazon rainforest when they are shot down by a mysterious craft. They are captured by the Badan Empire, a global terrorist organization that claims to be the successor to every previous evil group the Kamen Riders had defeated. Shizuka is executed after learning too much of Badan's secrets, while Ryo, due to his exceptional physical condition and skills as a pilot, is forcibly transformed into a powerful cybernetic soldier known as a Combat-Roid. His memories are erased, and he becomes the ruthless agent designated as ZX.

While on a mission for Badan to transport a rare and volatile energy source called Badanium 84, an accident causes Ryo to regain his memories. Horrified by what he has become and driven by a thirst for revenge against the organization that killed his sister, he escapes. Ryo begins a one-man war against Badan, targeting their shipments of Badanium 84, which is the crucial fuel for their ultimate doomsday device: the Space Break System, a weapon capable of tearing apart the fabric of time and space itself.

During his attacks on Badan's convoys, Ryo encounters several other strange warriors, whom he initially mistakes for enemies. These are the Kamen Riders, previous cyborg heroes who, like him, were transformed against their will and now fight for justice. The original nine Riders, including Kamen Rider V3, Riderman, and Kamen Rider Super-1, recognize Ryo not as a threat but as a brother-in-arms. They show him video records of their own origins and battles, revealing to Ryo that he is not alone and that his power can be used to protect rather than destroy. Accepting his fate, Ryo takes on the title of Kamen Rider ZX, the 10th and newest member of their ranks.

Together, the ten Riders launch a full-scale assault on Badan's hidden base. Ryo is confronted by his former friend and comrade from his time as a brainwashed soldier, Eisuke Mikage, who willingly became the Combat-Roid Tigeroid because he believes only power matters. Ryo is forced to destroy his friend in a final duel. The Riders then battle Ambassador Darkness, the commander of Badan who seeks to avenge his brother, Ambassador Hell, who was defeated by the original Kamen Rider. When Ambassador Darkness activates the Space Break System, the ten Riders combine their energy into a single devastating technique called the Rider Syndrome. Channeling this immense power through ZX, they empower his finishing kick to destroy Ambassador Darkness and obliterate Badan's base. As the base collapses, the giant, skull-like visage of the Great Leader of Badan appears, laughing at the Riders before vanishing, suggesting the evil is far from truly destroyed. Having proven his courage and found a new family, Ryo Murasame stands united with the other nine warriors as the newest hero bearing the title of Kamen Rider.