The Underground Empire Badan makes its move, intending to invert the world. Kouta and Mai are pulled through one of these inversions where the discover a boy named Shu, who also holds the power to invert the world. As the Riders fight to protect Shu, Fifteen appears again, seeking Shu. As Kouta flees, past Riders gather and fight one another, split between the Showa and Heisei era Riders. But, the truth of Badan is not as it seems…

The evil organization Shocker alters history by sending their newest creation, Kamen Rider 3, to defeat Kamen Riders 1 and 2 back in 1973. In the alternate present, Shocker is more powerful than ever, and most of the Kamen Riders are now either imprisoned or brainwashed to work for them. One such brainwashed Rider is Shinnosuke Tomari who thanks to his partner Kiriko Shijima, remembers the truth and joins forces with the remaining Kamen Riders who are still at large to make a stand against Shocker and put history back on its proper course.

The story begins with a regular day with Takumi, Mari and Keitaro, which suddenly turns into a musical number thanks to a mysterious new boom box created by Smart Brain. As it turns out, Yuji, Yuka and Naoya have the same boom box and are greeted by a rapping Smart Lady who reveals that it's part of a new plan by Smart Brain to invoke people into singing all over Japan. The reason: To get the three Rider Belts back!

Smart Brain, once Japan's largest company, lost its power due to repeated internal conflicts and was placed under government control with the goal of exterminating Orphnochs. Mari Sonoda, who is living with Orphnochs at the Western Laundry Store Kikuchi, when Takumi Inui, who had disappeared several years earlier, reappeared.