The Double Riders are what the first two Kamen Riders are known as collectively.
Before being transformed into the powerful hero, Kamen Rider 1, Takeshi Hongo (本郷 猛 Hongō Takeshi) was a college biochemistry student. Born in August 15, 1948, he is an intelligent young man with an IQ of 600 who attended Jounan University and enjoyed riding motorcycles. Hayato Ichimonji (一文字 隼人 Ichimonji Hayato), born October 10, 1949 is a free-lancer photographer, was kidnapped by the organization Shocker in order to become a reconstructed human able to fight the deserter Takeshi Hongo, aka Kamen Rider.
While practicing for the grand prix, Hongo followed a group of bikers who were calling him out, only to fall into a trap sent up by terrorist organization Shocker as they converted him into a super-powered cyborg before he woke up a week later as they were about to complete the process with brain washing. However, Hongo managed to escape Shocker's base by chance due to his college professor Midorikawa, who had been working for Shocker and was the man who selected Hongo to join the organization. The two were pursued by the Shocker Inhumanoid Spider Man and his forces as they hide out at warehouse 55 at the harbor. Unfortunately, Spider Man managed to track Ruriko to their location and killed the Professor as Ruriko arrived, blaming Hongo for her father's death before Spider Man took her. Pursuing them, Rider 1 fought through Spider Man's minions before battling the Inhumanoid, killing him with a Rider Kick.
As the Kamen Rider, Hongo battled Shocker's monsters before learning that Shocker recreated the original Kamen Rider cyborg design on Hayato Ichimonji in an effort to combat the threat that Hongo posed to their organization. Hongo helped Ichimonji escape from Shocker, leaving him to fight Shocker as Kamen Rider 2 while he left the country to fight Shocker's other branches around the world. Eventually, Hongo returned to Japan and, as Kamen Rider 1, joined forces with Ichimonji as the "Double Riders" to finish Shocker and then destroy the Gel-Shocker organization that formed soon after.
Kamen Riders 1 and 2 next appeared in the sequel Kamen Rider V3. The titular V3 was Shiro Kazami, a junior of Hongo's, who begged them to turn him into a Kamen Rider after the murder of his family by the Destron organisation. Despite initially refusing, when the young man was mortally wounded rescuing the duo from a deadly trap, the two Riders transformed him into Kamen Rider V3, possessing both Kamen Rider 1's "skill" and Rider 2's "strength". The Double Riders were apparently killed detonating an atomic bomb, but reappeared alive and well later on in the series. They then rejoined the battle with their junior and later Riderman as they managed to defeat Destron across the world.
The Double Riders would make appearances in most succeeding Kamen Rider TV series, with the exception of Kamen Rider Amazon, Kamen Rider Super-1 (although they show up in the Super-1's movie) and Kamen Rider Black. Kamen Rider 1 would lead the senior Riders once again in the final episodes of Kamen Rider Black RX.
Between Kamen Rider Kuuga and Kamen Rider Kiva, no past Riders appeared in the new live action productions of the franchise. However, in 2009, Kamen Rider 1 reappeared alongside the other Showa Riders to assist Kamen Rider Decade in the movie Kamen Rider Decade: All Riders vs. Dai-Shocker. Kamen Rider 1 led the Riders from the Showa and Heisei periods in the final battle against Dai-Shocker and, alongside Kamen Rider 2, defeated Dai-Shocker's Ambassador Hell, with help from Kabuto and Black RX. Rider 1 went on to help defeat Shadow Moon and King Dark.