After Natsumi Hikari has a nightmare of every Kamen Rider including Kuuga in his fearsome Ultimate Form, being defeated by a figure called Decade, she helps her grandfather Eijiro deal with unsatisfied customers. The photographer, Tsukasa Kadoya, seems to be at fault, but he simply tells Natsumi the photos are bad because he does not belong in this world. After the argument, various holes in reality open, allowing the Orphnoch, Undead, Makamou, Worms, Imagin and Fangires to start terrorizing their world. In the bedlam, she finds a corroded belt and book, recalling it as Decade's from her dream. She gives Tsukasa the belt and book, allowing him to transform into Kamen Rider Decade by using a card from the book and inserting it into the belt. Using other cards, he fights the various monsters that have invaded his reality by transforming into Kamen Riders Kabuto, Faiz, and Hibiki. Back in his own reality, Tsukasa is warned by Wataru Kurenai that his reality is being superseded by the nine previous Kamen Riders' realities. He must go to their worlds and defeat the other Kamen Riders to save his own world, while Wataru keeps it frozen in place. Back in the Hikari Studio, Eijiro reveals a large photo that shows another world. Stepping outside, Tsukasa finds himself in this world as a police officer while Kamen Rider Kuuga appears to aid the police against two Gurongi.
Recap / Kamen Rider Decade Ep 1 Rider War