Heisei Rider vs. Shôwa Rider: Kamen Rider Taisen featuring Super Sentai (2014) on Netflix

Plot summary

When workers of the Yggdrasill Corporation are dragged underground in Zawame City, Kota Kazuraba and Mai Takatsukasa investigate and fall into a hole that leads to a buried version of the city. They meet Shu, a boy who can revert things and cannot remember what important task he has to do. Kota and Mai bring him to the surface, where Takeshi Hongo demands they hand the boy over and soldiers of the Badan Empire arrive to seize him. As Badan attacks, Kota fights as Armored Rider Gaim beside Baron, Ryugen and Zangetsu Shin, and Kamen Rider Fifteen, who holds a Lockseed with the powers of the fifteen Heisei Riders, comes for the boy too. Narutaki appears and states that the fifteen Heisei Riders must unite against the Badan Empire, warning that the Shōwa Riders will not help them.