Upon arriving in the World of Ryuki, Natsumi goes to the Atashi Journal headquarters to learn about the world's Kamen Riders from its editor, Reiko Momoi. After Natsumi's arrival at the journal's offices, Reiko is mysteriously murdered, leaving Natsumi as the culprit as she had a small fork in her hand. Natsumi is arrested under suspicion of murder, and is to be sentenced with a Kamen Rider Trial: a system where Kamen Riders fight in the Mirror World to determine the outcome of criminal cases. Natsumi calls upon Tsukasa to be her lawyer, who meets with Shinji Tatsumi to hear what he knows of what happened to Reiko. As Shinji reveals that he is Kamen Rider Ryuki and he believes Natsumi was framed, his former friend Kamen Rider Knight is fighting in the Kamen Rider Trial, defeating several opponents. Shinji suggests to Tsukasa that he should fight as Decade in the Kamen Rider Trial to clear Natsumi's name.

After defeating Knight, Decade is about to finish him off when Ryuki pleads with him not to deliver the final blow, as he feels Knight must answer for his crime by his hand. However, Abyss uses this time to make his move to take out all three Riders at once, forcing them to retreat with Ren refusing to answer Shinji as he leaves. After her brief encounter with Narutaki, Natsumi learns that Shinji and Tsukasa are convinced that Ren is Reiko's murderer, although Yusuke sees otherwise. Meanwhile, a wounded Ren battles Kamen Rider Odin and defeats him, emerging from Mirror World with Odin's Time Vent as Shinji and Yusuke find him. Learning that Ren obtained the card to save Reiko, Shinji uses the card to go back in time as Tsukasa holds Abyss off. Arriving seconds before Reiko's death, Shinji and Tsukasa manage to save Reiko with the latter cornering her true murderer, Kamata.