The Hikari Studio arrives in a new A.R. World where, after seeing that Tsukasa's photos of this world are coming out perfectly, Natsumi realizes they may be in Tsukasa's home world. Tsukasa realizes he remembers the mansion depicted on the backdrop that took them here. Finding the manor, Tsukasa opens the door as he Natsumi and Yuusuke meet a girl named Sayo,who identifies Tsukasa as her older brother. As Sayo learns that Tsukasa lost his memories, revealing how he left her a year ago, her caretaker Nobuhiko Tsukikage arrives and confronts him about leaving his sister alone for so long. But when the Nine Worlds fusion starts to influence the World of Decade, Tsukikage reveals that Tsukasa's journey has now enabled a solution to the crisis by determining the strongest Kamen Rider in existence: The Rider Battle. When his memories return, Tsukasa gives Tsukikage the clearance to make it happen. Three days later, after numerous matches, it comes down to a 6-man tag team match between the team of Decade, Kuuga, and Diend vs. V3, BLACK, and Super-1. With Diend running off and Kuuga wounded, Decade is forced to defeat all three on his own. Upon victory, a castle emerges from the stadium with Yusuke and Natsumi finding themselves in the throne room surrounded by various monsters as they are welcomed to Dai-Shocker before Eijiro appears as he suddenly reveals himself to be Doctor Shinigami, toasting to their leader's return: Tsukasa. Revealing all of their actions were to ensure Dai-Shocker would achieve its goal of dominion of all the saved worlds, Tsukasa sends the horrified Natsumi and Yusuke into the sewers.