Based on

Movies like Kamen Rider Zangetsu the Stage -Gaim Gaiden- (2019)

Takatora Kureshima (Kamen Rider Zangetsu) visits a former Yggdrasill Corporation experimental project site located in the poverty and conflict stricken Republic of Torukia. He is visiting the site for the first time in 8 years, and is suddenly attacked! Suprised he stumble and falls into a giant hole which leads into an underground city. This underground city is the most dangerous place in the republic, in which many young men are killed defending themselves while trying to survive. These men use Lock Seeds and Drivers to transform into Armored Riders. Due to the fall Takatora has lost his memory. He is helped by the leader of Team Orange Ride. Unbeknownst to them, there are cameras hidden all around the underground city. The ones behind the cameras are responsible for the conflicts in the city. One man in particular is seeking revenge on Takatora for a specific reason...