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I'm assuming the rider war comes after the movie.
Blade (Kenzaki) comes back to tell Tsukasa that with Daishocker gone, the interdimensional problems of villians crossing and corrupting is now over. However, there is a final task he must take on since he is the only one to cross various worlds.
He is given the task of destroying (wiping out of existence) all of the ALTERNATE riders created by the interdimensional shifts. The reason for this is that there cannot exist 2 of the same riders on the same plane of existence. For example, there cannot be 2 Kamen Rider Blades, Kivas, Ryuki, etc. As the original riders existed first, it is only right that the Alternate riders be removed from existence to stabilize reality. Tsukasa is uneasy with this.
Tsukasa hears this and feels great remorse about the task as it means that he has to destroy all the allies he had met along the journey including Yuuske. We see the alternate riders arriving at this central rider world with each part of the world segmented to represent each of the alternate rider, i.e. a mesh of various worlds stuck together in one piece.
Tsukasa meets the Kid Wataru and Kid Asumu and he tells them the whole existential idea and they accept the consequence as they know about sacrificing for greater good (i.e. Wataru accepting responsibility as Kiva, Asumu killing Hibiki) and want to assist Tsukasa.
Apollo Geist, who is the only survivor of the Daishocker empire, goes to talk to Alternate Blade and convinces him that Decade is trying to kill them and that the Alternate riders should be the ones to exist and not the originals. Apollo plans to start a new empire and wants to use the Alternate riders as his pawns, he tries to gain power through each of the worlds. (i.e. Become King in Kiva World). Apollo Geist is the one that rallies all of the alternate riders to destroy Decade and then to destroying the original riders and we have the start of the rider war for their right to exist as people.
Eventually, Yuuske catches on and cannot believe that Decade is willing to destroy his fellow friends that he made along his journey, he cannot take the betrayal and that is why he is the most furious about fighting him. Basically we get an epic fighting battle and screaming match between Yuuske and Tsukasa.
Tsukasa eventually kills all the riders and leaves Yuuske last, he cries about killing him, but Yuuske realizes it is for the sake of humanity and goes willingly. Tsukasa goes into the dazed crying state. A new monster appears and we go into Kamen Rider Double.
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