To investigate a mysterious incident, Kuzuba Kota (Gaku Sano) sneaked into the strange and dangerous Heim Underworld Forest to investigate. On the way, he witnessed a teenager being attacked by a monster from the Badan Empire. When he was about to transform into Gaim, a man who looked like a samurai suddenly appeared, shouted "Transformation", and then appeared in the image of Kamen Rider No. 1. It turned out that he was the legendary man Hongo Takeshi (Hiroshi Fujioka)! Kamen Rider No. 1 solved the monsters around him at a lightning speed, and the two generations of justice warriors in Showa and Heisei met, but Kota was full of confusion about this legendary Kamen Rider who was about the same age as his father. During this period, Kamen Rider Decade Kadoya Shi (Masahiro Inoue) was actively contacting 15 Heisei Kamen Riders in the world, and he also got in touch with Kota. The successive terrorist incidents were obviously caused by the Badan Empire, and their goal was to destroy all mankind. The Heisei Knights who were actively preparing for the battle encountered the Showa Knights led by Hongo Takeshi. Hongo Takeshi believes that the root cause of the flooding of the Badan Empire is all related to the Heisei Kamen Rider. Two generations, two generations of warriors, each side holds a different view and refuses to give in to the other. Eventually, the conflict escalates and the war between the Kamen Riders breaks out. At the same time, the evil plan of the Badan Empire is still unstoppable. With the addition of two other justice teams, Resha Sentai Tokugawa and Zyuden Sentai Kyoryuger, the confrontation between the justice warriors becomes more complicated and confusing...