With only six combatants left in the Rider War, Shiro Kanzaki feels time running out to save his sister Yui, whose life is quickly fading as her twentieth birthday approaches. The setting shifts between the real world and the Mirror World, a dangerous dimension where Kamen Riders fight using monstrous Contract Beasts. Yui shows Shinji Kido and Ren Akiyama her childhood drawings, revealing the origin of the Mirror Monsters and explaining why Shiro started the deadly conflict.

Shinji, the idealistic Kamen Rider Ryuki who wants to stop the fighting, encounters Miho Kirishima, a young woman who cons wealthy men for money. After chasing her down, he discovers she is Kamen Rider Femme, the first female Rider in the series. Miho has entered the Rider War for two reasons: to avenge her sister, who was murdered by the sadistic Takeshi Asakura, Kamen Rider Ouja, and to use her wish to bring her sister back to life. Despite Shinji’s pleas to abandon the war, Miho remains determined.

As the battles escalate, Asakura as Ouja overpowers Femme and summons his three monsters to form Genocider, preparing to destroy her. Before he can strike, a black version of Kamen Rider Ryuki called Ryuga appears and attacks Ouja, destroying Genocider and reverting Asakura to his blank Rider form. Femme then shatters Asakura’s Advent Deck, leaving him unable to survive in the Mirror World, and he disintegrates. Meanwhile, Shuichi Kitaoka, Kamen Rider Zolda, chooses to abandon his Rider duties and spend his remaining time dating Reiko Momoi instead.

Miho, believing Shinji saved her, treats him to dinner. While Shinji is in the bathroom, his mirror image emerges into the real world. Disguised as Shinji, this doppelganger attempts to throw Miho off a rooftop. When she realizes the deception, he reveals himself as Ryuga. Miho transforms into Femme to fight him. Ren Akiyama, as Kamen Rider Knight, sees the battle and mistakes Ryuga for Shinji. Femme is mortally wounded, but Shinji arrives as Ryuki and saves her, catching a glimpse of his dark reflection before Ryuga escapes.

After leaving the Mirror World, Shinji and Miho share a final moment together before she dies from her wounds. Ren confronts Shinji, believing his friend has finally understood the meaning of the Rider War and demanding they fight. Shinji refuses and remembers his childhood: he had known Yui as a child but abandoned her, which caused her to create a doppelganger of him in the Mirror World. This reflection became Ryuga, a darker version of Ryuki. Overwhelmed with guilt, Shinji allows Ryuga to trick him into merging their bodies so the mirror version can become a real human and take over.

Ren witnesses this and transforms into Knight to battle Ryuga. As they fight, Yui’s condition worsens. On her twentieth birthday, Yui refuses the new life her brother has tried to give her through the Rider War and takes her own life. Shiro screams in anguish, causing all the mirrors in the city to shatter. Ryuga attempts to finish off Ren but is distracted by Yui’s body. Shinji struggles from within, separating from Ryuga. The two face each other in a climactic battle, which ends with Ryuki destroying Ryuga with his Final Vent. With the war seemingly over, Ren tells Shinji he must win, and Shinji agrees to fight willingly. Before they can battle, they transform into their Survive forms and turn to face an approaching horde of Hydragoon Mirror Monsters as the movie ends.