The moment the mysterious box "Pandora's Box" found on Mars was opened, a huge wall suddenly appeared! The story begins here. The wall called "Sky Wall" divides the Japanese archipelago into three parts, and the capitals of "Eastern Capital", "Western Capital" and "Northern Capital" were born from it. Among them, an unidentified life form called Smash appeared in "Eastern Capital" and secretly forced humans into a desperate situation. Standing in front of Smash is a young man with a belt on his waist and a mysterious bottle in his hand. "Transform!" - The young genius physicist transforms into Kamen Rider Build and stands in front of Smash!