After the civil war that tore apart the regions of Touto, Hokuto, and Seito, the new governors of each region work together to reunify Japan. In reality, however, they are part of an organization called "Blood Tribe", which intends to take over the entire world. Considering Sento Kiryū/Kamen Rider Build as a major threat against their plans, the Blood Tribe begins their plan by somehow manipulating his friends and other civilians to attack him. With even Ryuga now against him, Sento must now fight to survive while trying to stop the Blood Tribe's plan from succeeding.