The mysterious box "Pandora Box" brought back from Mars by a planetary exploration vessel. The energy released from the box created a huge wall, and Japan was divided into three parts. And 10 years later. It is rumored that there is a Kamen Rider Build that fights to protect people in the city of the eastern capital where the mysterious monster "Smash" is dark. The true identity is Kiryu Sento, a genius physicist with memory loss. The battle that unravels the mystery of Pandora Box and regains its own memory begins.