The story follows Tsukasa Kadoya, an amnesiac photographer in the World of Natsumi. During an attack of many different Kaijin from throughout the Heisei Kamen Rider history Tsukasa becomes Kamen Rider Decade. He then learns that he needs to save the World of Natsumi by traveling to the nine AR worlds, meaning other rider worlds or alternate reality worlds. He begins traveling through the worlds with his friend Natsumi and her grandfather. However, he later begins traveling with Yusuke Onodera from the World of Kuuga, Kivala from the World of Kiva, and Daiki Kaito from the World of Diend, who can transform into Kamen Rider Diend. While journeying through the worlds Tsukasa and his companions meet Narutaki, a man who believes Tsukasa is the destroyer of worlds. They also begin running into Dai-Shocker, an alliance of terrorist organizations from across the many worlds. Will Tsukasa and his companions save the many worlds and stop Dai-Shocker, or will Tsukasa become the prophesied destroyer of worlds?
