Mystery of Naming (ネーミングの謎, Nēmingu no Nazo) is the first episode of Kamen Rider Zi-O: Supplementary Plan. It was released on TTFC accompanying the first episode of Kamen Rider Zi-O, and is referred to as "episode 1.5" of Zi-O. Kamen Rider Zi-O: Supplementary Plan (仮面ライダージオウ 補完計画, Kamen Raidā Jiō Hokan Keikaku) is a TTFC-exclusive miniseries serving as supplementary material to the first 16 episodes of Kamen Rider Zi-O.
