Thirteen Advent Card decks were created for thirteen Kamen Riders. They make Contracts with monsters from the mysterious Mirror World, a parallel dimension opposite to our own in which only the Kamen Riders can exist. The Riders draw on their monsters' strength in exchange for feeding them the life force of the creatures they destroy. The creator of the Advent Cards has only one rule: that there can be only one Kamen Rider. The others must be killed, and the sole victor will be granted a single wish, which leads to a conflict known as the Rider War.
Shinji Kido is a carefree, inexperienced reporter for the ORE Journal, becomes Kamen Rider Ryuki in order to protect people from the monsters living inside the Mirror World and stop the senseless war among Riders that Shiro Kanzaki has started. Each Rider (except for Shinji), was lured into the Rider's battle with the promised of having his wish come true if he is the last one standing. Among the chaos Shinji somewhat befriends Ren Akiyama (Kamen Rider Knight) and Kanzaki's sister Yui.