Long ago, the Twelve Animal Signs formed the Zyunishi Doumei (The Twelve Animal Signs Alliance) to protect humanity from mysterious lifeforms known as FUKASHIGI. Preparing for an inevitable battle against FUKASHIGI, the members of the Zyunishi Doumei entered a 100-year slumber. In the present day, Sora Otosumi serves as the personal Meister of the Mizuki family. To protect the family's daughter, Yuna Mizuki, he transforms into Kamen Rider MY-TH and joins the fight against FUKASHIGI. But when Sora comes into conflict with the Twelve Animal Signs over FUKASHIGI, a new Kamen Rider “myth” begins!

As Tatsuomi, who had infiltrated the Mizuki household as a tutor, continued his investigation, a new Fukashigi emerged. Sora transforms into My-Th, but just then, Datt appears. Datt transforms into Kamen Rider Datt and, ignoring the astonished My-Th, launches an attack on the Fukashigi. Datt uses her exceptional jumping ability to corner the Fukashigi, but My-Th intervenes, turning it into a three-way battle...!?

Yuna meets Shiori, a girl whose father has gone missing in the city, and deduces that it’s the work of a Fukashigi. Sora also begins his own investigation. Tatsuomi, who had been keeping an eye on Sora, witnesses Sora transform into My-Th right in front of the Fukashigi. Seizing the moment, he pulls out his own Driver and transforms into Kamen Rider...!

"If we don't have that, Yuna's in big trouble!" Tatsuomi is suspicious of Sora, who is unusually open about his emotions. Just then, Yuna sends them new Fukashigi information. Sora transforms into My-Th and challenges the Fukashigi, but Jyao, who is after My-Th’s transformation device, intervenes...!