Masked Rider, aka Takeshi Hongo, is an altered human. He was altered by Shocker, planning an evil world conquest at the secret society. Because Masked Rider is a human of freedom, he will fight against Shocker! (仮面ライダー本郷猛は改造人間である。彼を改造したショッカーは世界制覇を企む悪の秘密結社である。仮面ライダーは人間の自由のためにショッカーと戦うのだ!, Kamen Raidā Hongō Takeshi wa Kaizō Ningen de aru. Kare o Kaizō shita Shokkā wa Sekaiseiha o Takaramu Aku no Himitsukessha de aru. Kamen Raidā wa Ningen no Jiyū no Tame ni Shokkā to Tatakau no da!?) (in episodes 14–52, the name Takeshi Hongo was replaced by Hayato Ichimonji).
The story took place in a world plagued by Shocker, a mysterious terrorist organization. To assist its plan in world domination, Shocker recruited its agents through kidnapping, turning them into mutant cyborgs and, ultimately, brainwashing them. However, one of the victims was rescued just before the final step, and his name was Takeshi Hongo (本郷 猛, Hongō Takeshi?). With his sanity and moral conscience remaining intact, Hongo assumed the identity of the grasshopper-looking altered-human (改造人間, kaizō ningen?) superhero Kamen Rider and battled Shocker with his newly acquired powers.