謎の白い服の青年がショウマの前に現れ、彼を追っていた追跡者とガヴに変身して戦うが、突如として現れた謎の扉を開くことで現れた「おカシな世界」。そこは元の世界に似ているが、闇菓子が存在せず、その世界の幸果や絆斗、ラキアやストマック家の兄姉も異なる人生を歩んでいた。
ショウマはおカシな世界のはぴぱれで働くタオリンという過去の記憶が一切ない青年に出会う。そんな中、人間やグラニュートとは異なる種族・ミューターのキングを名乗るカリエスが無差別に街を襲撃し、未知のゴチゾウで仮面ライダーカリエスに変身する。
ショウマとタオリンは、巨大なお菓子の家を発見するが、タオリンはかすかにこの場所を覚えていた。
A mysterious door suddenly appears in front of Shoma and Sachika. Shoma is dragged through a door and arrives at a peaceful world where Dark Snacks don't exist!? There is also a Hapipare in that world, where a young man with a gavv named Taorin works. However, he has no memory of any of his past. Shoma helps Taorin to recover his memories. Meanwhile, a man calling himself "Caries" begins to attack the town indiscriminately, turning the peaceful world upside down! As the two struggle to restore peace, they arrive at a giant house of sweets where they were about to grab something, but Caries transforms himself with an unknown Gochizo and hunts down Shoma and his friends! What is the source of the overwhelming power that the strongest enemy has acquired? Kamen Rider Gavv's great adventure beyond time and space begins now!
