A parallel dimension known as the Mirror World exists as a ghostly reflection of our own reality, accessible only through reflective surfaces. Within this dangerous realm, monstrous creatures known as Mirror Monsters feed on human life force, dragging unsuspecting people through mirrors to their deaths. The mastermind behind a deeper conspiracy is Shiro Kanzaki, a mysterious figure who has orchestrated a deadly contest called the Rider War. He has distributed thirteen Advent Decks, each granting a chosen individual the power to form a contract with a specific Mirror Monster. By transforming into armored warriors called Kamen Riders, these combatants are ordered to fight one another to the death. The last surviving Rider will be granted one wish, anything they desire, a prize Shiro intends to use to save his dying sister, Yui.

The central figure thrust into this conflict is Shinji Kido, a kind-hearted but naive trainee reporter for the ORE Journal. While investigating a series of disappearances, Shinji stumbles upon an Advent Deck and is pulled into the Mirror World. There, he witnesses a battle between a humanoid bat monster and a powerful dragon named Dragreder. He is saved by Ren Akiyama, a cold and pragmatic fighter who transforms into Kamen Rider Knight. Ren fights with brutal efficiency, driven by a singular goal: to win the Rider War and revive his comatose fiancée, Eri Ogawa. Despite Ren's warnings to stay out of the fight, Shinji is horrified by the senseless violence and the threat to innocent lives. He forms a contract with Dragreder to become Kamen Rider Ryuki, determined not to win a wish, but to stop the Riders from killing each other and to protect humanity from the Mirror Monsters.

The series unfolds as a battle royale across the city of Tokyo, with the combatants ranging from the honorable to the psychotic. Shinji and Ren form an uneasy and often argumentative alliance, with Shinji boarding at the mysterious owner’s antique shop-café Atori where Ren frequently hides out. They are soon confronted by a growing roster of Riders, each with their own agendas. These include the ruthless lawyer Shuichi Kitaoka, Kamen Rider Zolda, who fights to stave off his own impending death; the arrogant and sadistic high school student Takeshi Asakura, Kamen Rider Ouja, who revels in chaos and violence; and the gentle idealist Miyuki Tezuka, Kamen Rider Raia, who tries to destroy the Rider system itself. As the war escalates, Shinji discovers the tragic origin of the conflict, learning that Yui Kanzaki, who helps Ren and wants to find her brother, is fated to disappear on her twentieth birthday. Shiro started the Rider War to use its wish to erase her from the Mirror World and allow her to live as a real human.

Over the course of fifty episodes, the Rider War exacts a heavy emotional toll. Shinji struggles to find a peaceful solution, often coming into conflict with Ren's ruthless pragmatism. Key alliances form and shatter, and several Riders meet their end, their wishes left unfulfilled. In the final narrative arc, the war narrows down to a handful of survivors. Yui finally confronts her brother, leading to her death and the weakening of Shiro’s control. In a climactic series of battles, Shinji obtains the Survive Rekka card, evolving into Kamen Rider Ryuki Survive to gain enough power to challenge the system itself. Ultimately, during a desperate fight to save civilians from a swarm of Mirror Monsters, Shinji Kido is mortally wounded. He dies in Ren’s arms, having proven that protecting others is more important than victory. Ren, now fighting with Shinji’s conviction as well as his own, defeats the final Rider, Kamen Rider Odin. With Yui gone and Shinji dead, Shiro realizes the futility of his plan and uses his remaining power to reset time, erasing the Rider War entirely. In the new timeline, all the deceased Riders are alive, leading peaceful lives unaware of the conflict, while Yui Kanzaki finally fades away peacefully, and Shiro disappears. Ren finds work at the Atori coffee shop, and Eri awakens from her coma, but he carries the quiet memory of the friend he lost.