Only the people who have been into Human Experimentation have resurrected their memories of the Old World in the Sento Kiryuu's (Kamen Rider Build) New World. In the New World there were people who are planning to rule the world using the power of science as a Weapon. Their name is Downfall. These are the terrorist group who are planning to make use of Rider System.
Downfall attacks the Government Office which had approved the use of Ride System. Gentoku Himuro (Kamen Rider Rogue) who is a high ranking official gets attacked by an enemy who have the ability to stop transformation into Kamen Rider. Thier evil hands reach Sento, Ryuga Banjo (Kamen Rider Cross-Z), Kazumi Sawatari (Kamen Rider Grease) & his farm. Rider System gets taken away. They also aim to take away White Panel and take Misora Isurugi as a hostage.
Even if so there were people who wouldn't give up.
It was Kazumi a.k.a. Kashira & his supporters the Three Crows who love Misora more than anyone else. They begin a coutnerattack.
It was upto Kazumi to save Misora because he was the only one who hasn't lost his ability to transform. In order to defeat the enemy, he has to undergo a transformation which will surpass his enemy. For this there was a need to get the element from the Three Crows. Using this they can create a Item which will help him to transform into his Ultimate Form. This experiment will lead them to lose their life.