The cynical butler, Sora Otosumi lives alone in a mansion with the innocent and carefree young lady, Yuna Mizuki. The two lead elegant lives, but they harbor a secret. Otosumi transforms into Kamen Rider MY-TH to fight mysterious monsters known as Fukashigi, and effortlessly defeats these creatures like a mouse. Around the same time, the Zyunishi Doumei (The Twelve Animal Signs Alliance), who also fight the Fukashigi, awaken from their 100-year slumber. Their leader is none other than a cat?! The alliance has begun to realize that their prey is being stolen. The battle between mouse and cat begins now!