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 that to protect his own world he must travel with Natsumi and her grandfather to the other Kamen Riders' Worlds called A.R. Worlds (Another Rider's World or Alternate Reality World) and eliminate the anomaly in that world. 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.