The story revolves around the nine previous universes of the Heisei Kamen Riders merging into one, something that would destroy all the worlds. To prevent this, Tsukasa Kadoya transforms into Kamen Rider Decade and is told to protect his own world. He must travel with Natsumi and her grandfather to the other Kamen Riders' Worlds (referred to as an A.R. World or Another Rider's World) and eliminate the anomaly in that world. Along the way, confronting the mysterious Narutaki and the thief Daiki Kaitoh, they are joined by Yusuke Onodera of the World og Kuuga and Kivala of the World Kiva.