During a motocross practice, Keisuke Jin is ambushed by two Myth Cyborgs sent by G.O.D., the Government of Darkness, a secret organization attempting to seize control of Japan. Forced to transform into Kamen Rider X to defend himself, Keisuke defeats his attackers, unaware that the entire battle is being recorded and analyzed by King Dark, the giant robotic commander of G.O.D. King Dark uses this data to create a powerful new super-cyborg warrior designed specifically to destroy Kamen Rider X.

The leader of G.O.D. has his scientists develop a fearsome monster known as Frankenbat, a hybrid creature combining the features of Frankenstein's monster with the vampiric abilities of a bat. Frankenbat begins terrorizing Tokyo by kidnapping women to drain their blood for survival, attempting to eliminate Kamen Rider X as part of King Dark's broader scheme. Meanwhile, other G.O.D. kaijin are attacking children all across the city, creating widespread panic before mysteriously disappearing.

As the crisis deepens, the original four Kamen Riders return to Japan from various locations around the world. Kamen Rider 1 arrives from New York, Kamen Rider 2 comes back from Paris, Kamen Rider V3 returns from Moscow, and Riderman travels from Tahiti. Despite Riderman having been presumed dead at the climax of the preceding Kamen Rider V3 series, he appears alive and well, introducing himself as Kamen Rider 4. Upon learning the full scope of King Dark's plans, these four veteran heroes join forces with Kamen Rider X to launch a counterattack against G.O.D. and their revived kaijin forces.

The five Riders unite to confront King Dark's monstrous enforcers, which include creatures inspired by mythology such as Cerberus, Cyclops, Icarus, and Ulysses, alongside the primary antagonist Frankenbat. Together, they fight to stop G.O.D.'s bloody scheme known as the Karakara Operation, a plan to drain the blood and water from all of Japan's citizens. The movie culminates in an alliance of the first five Kamen Riders leading a decisive assault against the forces of darkness.