A mysterious person restarts the Mirror World's Rider Battle, thus making Shinji Kido and other war participants regain their memories and transforming abilities. With the appearance of Another Ryuki, Sougo Tokiwa and Geiz Myokoin investigate this event, becoming involved in the war themselves...

"Mirror World" exists symmetrically beyond an invisible mirror in space, and an exact copy of our human world, where only monsters can dwell. Evil monsters frequently cross the mirror and attack our world. Only Kamen Riders who make a contract with a specific monster and obtain special power from it, can enter "Mirror World" and fight against evil monsters with the help of an "Advent Card." Shinji Kido, a journalist, contracts with a dragon and becomes Kamen Rider Ryuki. He fights a fierce battle with evil monsters. Many Kamen Riders appear one after another and fight to satisfy their own aims. However, only one Kamen Rider can outlive others. Thus, it is their fate to keep fighting to the last man.