Set after the events of the Japanese Civil War, the newly appointed governors of Seito, Hokuto and Touto now support each other in their mission to reunify Japan, however they are secretly members of Evolto’s home clan known as the Blood Tribe, who each have assumed government positions in order to take over the entire world. Being certain that Sento Kiryu/Kamen Rider Build is a major threat to their plans, they proceed with their Build Annihilation Plan (ビルド殲滅計画 Birudo Senmetsu Keikaku) to manipulate Sento’s friends and other civilians against him. With Ryuga Banjo now against him, Sento must find a way to fight and survive as he hunts for the truth to stop the Blood Tribe in succeeding with their ultimate plan.
但这一切，都只是为了消灭掉“假面骑士Build / 桐生战兔”，及将真相隐藏起来的完美计划。在失去了最好的拍挡下，从最恶劣的状况中所推断出的答案，就是奇迹的法则！
为何创造驱动器必须使用两枚瓶子呢？为何假面骑士Build / 桐生战兔得要战斗呢？答案，只有一个！“绝对可以的….。如果是我与你的话….。那么，最后的实验开始吧。”