Ten years ago, the Mars probe brought back the Pandora Box, which emitted a light that caused the ground to raise, and a massive wall to appear. Society has called this the "Skywall Tragedy". Japan has been separated into three sections, with three capital cities known respectively as Hokuto, Seito, and Touto. They were all in conflict with one another. It is now present day. In Touto, mysterious unidentified lifeforms called the Smash appear. Sento Kiryu secretly transforms into Kamen Rider Build, and he's sent day after day to fight against the Smash. However, Sento has no memories of the last 20 years. He only remembers, "A scientist in a gas mask, human test subjects, and a bat man". What on earth does all of this mean? Sento is employed at the Touto Institute of Advanced Matter Physics. While continuing to fight as Build, he will also be responsible for unraveling the mystery of the Pandora Box as a researcher. It's at this time that Ryuga Banjo makes a jailbreak in the middle of serving a prison sentence for murder. The Smash detected Ryuga and reacted to him. Sento tries to catch Ryuga, but Ryuga tells him an unexpected fact to try to prove his innocence.
