ケミーの捜索を続ける一ノ瀬宝太郎たちと、ふたたび出現したジャマトに立ち向かう浮世英寿たちの前に突如として現れた“レベル10の最強ケミー”≪クロスウィザード≫。彼の持つ不思議な魔法の力によって、桜井景和らがケミーの姿にされてしまった！さらには同時に5体もの“レベル10の最強ケミー”や、ギーツにそっくりな「ギーツケミー」も出現！相次ぐ異常事態に戸惑う宝太郎たちと英寿、そしてケミー化した景和たちは、それぞれチームを組み【最強のケミー捕獲ゲーム】に挑む！このゲームに隠された恐るべき目的とは。突如として現れた「ギーツケミー」の謎とは。仮面ライダーガッチャードと仮面ライダーギーツは、待ち受ける難関をクリアすることができるのだろうか……！
Houtaro Ichinose and his friends continue their search for the Chemies, and Ace Ukiyo and his friends confront the Jyamato, who have appeared again, when suddenly the "Strongest Level 10 Chemy", X Wizard appears. Its mysterious magical powers have transformed Keiwa Sakurai and the others into the form of Chemies! In addition, as many as five "Strongest Level 10 Chemies" and the Geats Chemy, which looks just like Kamen Rider Geats, will appear at the same time!
Perplexed by the series of unusual situations, Houtaro and his friends, Ace, and Keiwa, who has been turned into a Chemy, form teams to take on the "Strongest Chemy Capture Game"! What is the terrifying purpose hidden in this game? What is the mystery behind Geats Chemy's appearance? Will Kamen Rider Gotchard and Kamen Rider Geats be able to clear the hurdles that await them...?
