The world is breaking apart. The parallel universes of nine previous Kamen Riders are collapsing into each other, a cataclysmic merger that will end in the destruction of all worlds. At the center of this chaos stands Tsukasa Kadoya, a young man with a mysterious past and a talent for taking terribly out-of-focus photographs. Living above the Hikari Photo Studio, he is a drifter with no memories of who he is or where he came from, only a nagging feeling that he does not belong in this reality.

When creatures from across the Kamen Rider multiverse begin pouring into his world, Tsukasa is given a driver belt and a book of cards by Natsumi Hikari, the granddaughter of the photo studio’s owner. She has been haunted by a recurring nightmare known as the Rider War, a vision where a single figure called Decade defeats every Kamen Rider. Urged to fulfill his role, Tsukasa transforms into Kamen Rider Decade. To save his own dimension, he is told he must travel to the nine worlds of the Heisei era Riders, from Kuuga to Kiva, and defeat each of them to absorb their power. Joined by Natsumi and her grandfather Eijiro, who can transport their studio between dimensions, Tsukasa begins his journey.

Their first destination is the World of Kuuga, where they encounter Yusuke Onodera, a gentle but powerful young man who fights as that world's Kamen Rider Kuuga. Rather than destroying him, Tsukasa befriends Yusuke, who chooses to join the group on their travels. As the team moves from one alternate reality to the next, they discover that these are not the original stories, but "A.R. Worlds" where familiar events and characters are distorted. In the World of Kiva, the Fangires have made an uneasy truce with humanity. In the World of Ryuki, the Rider battle is a formalized judicial system of trial by combat. In the World of Faiz, the battle against the Orphnochs takes place within a competitive high school hierarchy.

Two recurring figures shadow Tsukasa on his journey. The first is Daiki Kaito, an interdimensional thief who transforms into Kamen Rider Diend. Wielding a gun-like driver, Daiki can summon other Riders to fight for him as he searches for a legendary treasure that will grant any wish. He is a rival and occasional ally, always pursuing his own mysterious agenda. The second is Narutaki, a frantic and vengeful man who constantly proclaims that Decade is the "destroyer of worlds" and that all the dimensional chaos is Tsukasa's fault. Narutaki rallies forces against Decade at every turn, determined to prevent the prophesied catastrophe.

As the series progresses, the group learns that the true enemy is Dai-Shocker, a massive alliance of evil organizations from every Rider world. Their goal is to merge all dimensions into a single universe under their control. Tsukasa eventually confronts the shocking truth about his own past: before losing his memory, he was the Great Leader of Dai-Shocker, the very architect of the dimensional collapse he is now trying to stop. This revelation forces him to reject his former role and fight against the organization he once led.

The narrative builds toward the Rider War, the apocalyptic event from Natsumi's dreams. As the barriers between worlds completely dissolve, Tsukasa faces the ultimate choice between destroying the other Riders to save his own world or finding another path. The final episodes reveal that Tsukasa's original plan as the Great Leader was not simple conquest but an attempt to reset the multiverse to save it from a deeper corruption. After defeating the renegade leaders of Dai-Shocker and a powerful version of Kamen Rider Kuuga known as Rising Ultimate Form, Tsukasa seemingly sacrifices himself to restore the separated worlds, fulfilling his role as the destroyer who must break everything to rebuild it. His fate remains ambiguous, allowing the story of the passing-through Kamen Rider to linger as a legend across the newly restored multiverse.