Nine universes of the previous Kamen Riders from the Heisei era are merging into one, something that would destroy all the worlds. To prevent this, Tsukasa Kadoya transforms into Kamen Rider Decade and is told that to protect his own world he must travel to all of the other Kamen Riders' worlds (Another Rider's worlds) and eliminate the anomalies in those worlds. Along the way, confronting the mysterious Narutaki and the thief Daiki Kaito, they are joined by Yusuke Onodera of the World of Kuuga and Kivala of the World of Kiva.
