One year after the battle days of Sword of Logos ended, Reika Shindai is preparing for her wedding. However, this happiness is unexpectedly abrupted when her brother Ryoga receives some disturbing details about the upcoming wedding.

Granute, secret lifeforms, kidnap humans for "Dark Market Snacks." Shoma, from another world, transforms with Gochizo, a sweet-eating monster, into Kamen Rider Gavv via a mouth-like belt to stop Granute's attacks and save people.

2022年——乃是忍者活跃期间的年代。
所谓的忍术，对于该年代的人们来说，那已经是个日常，但也是个充满了新的运动竞赛方式。
平常只是作为月薪制的工薪族生活，总是展现出被妹妹称为 “废物的是只擅长忍术吧” 的让人吃惊程度地狼狈不堪的样子的青年：神藏莲太郎，他的另一个身份是与暗之忍者军团·虹蛇相战斗的传说的忍者·假面骑士Shinobi。
这是一个暗中保护他人，斩除堕落者的年轻忍者故事。座右铭为——“忍字意为斩刃之心！”