A young journalist named Shinji Kido is pulled into a strange mirror world while investigating a series of disappearances. There, he is attacked by a monster called a Mispider and is saved by Kamen Rider Ryuki, whose real identity is a man named Koichi Sakakibara. Sakakibara is already mortally wounded and, unable to continue fighting, passes his Advent Deck to Shinji. After transforming into Ryuki and defeating the monster, Shinji escapes the mirror dimension and meets Ren Akiyama, a stoic warrior who fights as Kamen Rider Knight, and a mysterious young woman named Yui Kanzaki. They explain that the Mirror World is the battleground for a deadly war. Thirteen Kamen Riders have been contracted to fight each other to the death, with the last surviving Rider granted one wish. Shinji rejects this violent game and resolves to stop the Rider War, just as Sakakibara had wanted.

Armed with a list of the Riders' identities, Shinji attempts to reason with each participant, but his pleas are met with hostility, mockery, and greed. The Riders are singularly focused on their desires and see Shinji as an obstacle. These include the brutal and caged Takeshi Asakura, who transforms into Kamen Rider Ouja; the arrogant lawyer Shuichi Kitaoka, who becomes Kamen Rider Zolda; and the selfish playboy Jun Shibaura, who fights as Kamen Rider Gai. One Rider, Itsuro Takamizawa, transforms into the chameleon-themed Kamen Rider Verde and proposes a temporary truce among the other Riders. Their shared goal is to eliminate Shinji and his idealistic mission. Verde pressures Ren to prove his loyalty by killing Shinji, but Ren hesitates, having formed a grudging respect for the persistent journalist. This act of defiance makes Ren a target as well, and the two are forced to fight for their lives against a unified force of their rivals.

The conflict rapidly escalates. In the ensuing chaos, Riders like the gentle Miyuki Tezuka, Kamen Rider Raia, are killed by their power-hungry peers. As Shinji and Ren desperately fend off their attackers, the god-like Kamen Rider Odin appears. With seemingly effortless power, Odin destroys Shinji's Advent Deck, shattering his ability to fight as Ryuki. As the remaining Riders close in for the kill, a mortally wounded Ren pushes Shinji aside and takes a fatal blow meant for him. In his final moments, Ren acknowledges Shinji as a friend and gives him his own Advent Deck, the power of Kamen Rider Knight. Shinji transforms into Knight and, filled with grief and determination, makes a final stand.

As Shinji confronts the core mirror that sustains the war, the narrative presents two divergent conclusions. In one ending, Shinji chooses to continue Ren's fight, transforming into the more powerful Kamen Rider Knight Survive and preparing to battle the seven remaining Riders alone. In the other ending, Shinji destroys the core mirror itself, causing the Mirror World and the Rider War to collapse. He returns to a normal life, but the peace is brief; the haunting reflection of a Kamen Rider appears in a nearby mirror, and the deck of cards materializes in his hand once more, suggesting the battle can never truly end.