The cynical servant Sora Otosumi, lives alone in a mansion with the innocent and carefree young lady, Yuna Mizuki. The two lead elegant lives but harbor a dark secret… that is to defeat the mysterious race of monsters known as the Fukashigi. Sora transforms into Kamen Rider My-Th and defeats these mysterious creatures with his mouse powers.
Meanwhile, the members of the Zyunishi Doumei, all sworn to fight against the Fukashigi, have risen from their 100-year long slumber. However their leader is… a cat?!
The alliance has begun to realize that their prey is being stolen. And so the battle between cat and mouse begins now!