Natsumi Hikari has repeatedly been having dreams of the Rider War, where every single Rider from the Heisei-era attack one being, a warrior known as "Decade." In reality, she lives with Tsukasa Kadoya, a young man who doesn't seem to be able to call Hikari Studio home. One day, monsters from different dimensions begin to attack people, with the world starting to fall in chaos. Tsukasa, who had been separated from Natsumi, meets the mysterious Wataru Kurenai, who informs Tsukasa about his title as Kamen Rider Decade, the Destroyer of Worlds. Discovering that the nine universes of the Kamen Riders from the Heisei-era are merging into one, which would result in the collapse of multiverse, Tsukasa inherits the mission of traveling across these worlds to eliminate the anomaly in each, while embarking on a journey to find one world where he can call home.