On his first day at the school, a strange creature identified as a Zodiarts appears. Though he appears to have been knocked out while fighting off the Orion Zodiarts, Gentaro sees Kengo's arrival in the Powerdizer exo-suit and later follows him and Yuki to the Rabbit Hatch where the Fourze Driver, created by Kengo's father, is being kept. Gentaro takes the item after hearing Kengo speak of his difficulty in manning the Powerdizer, and then faces off against the Orion Zodiarts as Yuki instructs him on how to use the driver. Transforming into Kamen RiderFourze, Gentaro defeats the Orion Zodiarts, and he is begrudgingly allowed to keep the Fourze Driver in order to battle the Zodiarts menace. Inspired by Tomoko Nozama's presentation of an urban legends website Kengo found that details the Kamen Riders' deeds for the previous 39 years, Gentaro forms the Kamen Rider Club while taking up the title of Kamen Rider.
Eventually, his club grows in members as does his array of Switches to utilize as he faces off against the group distributing the Zodiart Switches to his classmates: the Horoscopes. The club is later joined by Ryusei Sakuta, a transfer student who transforms into Kamen Rider Meteor, and their teacher Mr. Chuta Ohsugi. As the battle increases, the Horoscopes sets their motions to their final stage of their master plan before Kengo discovers his true existence.
As the final confrontation begins, Gentaro learns that Kengo is the Core Child, a construct of Cosmic Energy created from the Core Switch that Rokuro and Gamou reversed engineered to create the Fourze System and the Zodiarts Driver. Though he helps Kengo cope with him leaving to see the Presenters, the alien race that sent him to Earth, Gentaro is forced to watch Gamou end Kengo's life. Because of a letter Kengo left behind, Gentaro resolves to show Gamou the error of ways. Though he succeeds in reaching through to him, Gentaro watches as Gamou's body begins to dissolve into stardust, just as he promises to meet the Presenters in the chairman's stead. Upon learning that Gamou restored Kengo to life, Gentaro reforms the Kamen Rider Club into the Space Kamen Rider Club, with the intent to befriend everyone in the universe.