Now, shall we begin the experiment?

10 years ago when the Pandora Box was discovered on Mars and brought to Earth, it's activated and creates a Skywall which divides Japan into Touto, Seito, and Hokuto resulting into untold chaos. Along with creating a complex power dynamic between the three regions, the event brings waves of monsters called Smash down on the country, who terrorize and attack the citizens. Sento Kiryu, an amnesiac taken in by a mysterious cafe owner named Soichi Isurugi who provides him with a belt called the Build Driver that allows him to turn into Kamen Rider Build, which he uses to fight the Smash on his own.