Kamen Rider Decade: All Riders vs. Dai-Shocker
Kamen Rider Decade, aka Kadoya Tsukasa, arrives in a "certain world." In this world, for some reason, the photos Tsukasa takes do not distort the image. Suddenly, a certain Western-style mansion comes to mind. Following faint memories, Tsukasa reaches the mansion, where he meets a girl who calls herself his sister, Kadoya Sayo. Tsukasa becomes convinced this is his own world. However, even in this world, the "collapse of worlds" is progressing. Sayo's servant, Tsukikage Nobuhiko, claims that the cause of the world's collapse lies in the Riders born in each world, and declares that to stop it, they must decide on a single "strongest Rider."
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