The terrorist organization Shocker, formed from the remnants of a Nazi scientific cabal, seeks world domination through the creation of cyborg soldiers and genetic monsters. Their most promising subject is Takeshi Hongo, a brilliant biochemist and champion motorcyclist with an IQ of 600 whom they kidnap and surgically rebuild into a grasshopper-human hybrid. Before Shocker can complete his brainwashing, Hongo escapes, using his new powers to fight the organization as the original Kamen Rider. When actor Hiroshi Fujioka broke his leg performing his own stunts, the series introduced a second hero, photojournalist Hayato Ichimonji, who is similarly captured and modified but rescued by Hongo to become Kamen Rider 2. Hongo departs to battle Shocker overseas while Ichimonji protects Japan until his eventual return.

The live-action film Kamen Rider vs. Shocker, directed by Minoru Yamada and released in March 1972, brings both Riders together in a standalone story. Professor Daidoji has invented the GX device, a revolutionary machine capable of manipulating gravity. Shocker, now led by the sinister Doctor Death, attacks the professor to steal his invention. When the plans prove incomplete, Shocker learns that the missing activation formula is known only to Daidoji's young daughter, Tamami. The organization kidnaps the girl, and Doctor Death offers to exchange her for the professor. Hayato Ichimonji, having learned of the situation, accompanies who appears to be Daidoji to the exchange in Hell Valley. However, the professor is actually Takeshi Hongo in disguise. The deception revealed, the two Riders battle an army of Shocker monsters, including the newly created Zanjio, that have been resurrected for the operation. After securing Tamami's release, Daidoji reveals the secret information is hidden inside a teddy bear he gave his daughter for her birthday. Shocker learns this and sends Zanjio to steal the bear, but Kamen Riders 1 and 2 pursue the monster and destroy him, protecting the GX device from falling into Shocker's hands.

The film marked the first time Kamen Rider 1 performed his iconic transformation pose on screen. The narrative fits within the larger arc of the 1971 television series, where Shocker operates through regional commanders including Colonel Zol, Doctor Death, and later Ambassador Hell. After repeated defeats, Shocker merges with the African organization Geldam to become the more powerful Gel Shocker, forcing the two Riders to unite again for the final episodes of the series.