my review =)

The film opens with Zawame City ravaged by a fight between the Heisei Riders and the Shōwa Riders, before going back in time to the week before, when workers from the Yggdrasill Corporation are dragged by an unknown force to underground. Upon hearing rumors about it, Kouta Kazuraba investigates the area with his friend Mai Takatsukasa, just to fall into a hole as well and find themselves in an underground, alternate version of the city. The two find a boy called Shu, who has the power to revert things and is certain that he has something important to do, but he does not remember what it is. Kouta and Mai then bring the boy to the surface and Takeshi Hongo appears demanding them to hand him over, when soldiers from the Underground Badan Empire appear to kidnap him as well and Takeshi transforms into Kamen Rider 1 to allow Kouta, Mai, and Shu to escape. Badan then launches an attack on the surface, and Armored Riders Baron, Ryugen, and Zangetsu Shin appear to fight back alongside Kouta, who transforms into Armored Rider Gaim as well to protect his friends. However, Gaim is attacked by Kamen Rider Fifteen, who is also seeking the boy and also possesses a Lockseed that allows him to transform into the Rider Arms of the 15 Heisei Riders, and when Kamen Riders 1, 2, and V3 appear to fight him, Shu uses his powers to escape with Gaim and Mai.