Unidentified creatures called Smash have appeared at Touto, Sento Kiryu turns into Kamen Rider Build to fight against them. One day, he saves a guy named Ryuga Banjo who escaped from a prison.
Because of helping Ryuga, the government chased Build as a wanted criminal. Sento guesses that he can obtain clues to the mysterious organization by finding true culprit who killed Katsuragi.
After getting another of his memories back, Sento investigates the connection between Faust's experiments and the Pandora Box; Ryuga tracks down Nabeshima's family and stands up to protect them from the enemy.
Sento discovers that Katsuragi was the founder of Faust, and approaches his mother to learn more about Katsuragi's intentions, and obtain more clues regarding his lost memories.
Sento and Ryuga discover that Katsuragi was involved in creating a Build Driver of his own, and his possible involvement with Sento widens the distrust between them.
After learning from Sawa about the connection between Faust and Nanba Heavy Industries, Sento and Ryuga confront them to retrieve the Pandora Box and their stolen Full Bottles.
Sento and guys were driven into a situation where they had to hide Pandora Box in a old warehouse. They make a decision to uncover a government conspiracy.