10 years after the Pandora's Box disaster divided Japan into three separate nations, unidentified lifeforms called 'SMASH' have appeared.
To fight them, a prodigy physicist with no memory utilises the Full Bottles to transform himself. This man is Sento Kiryu, Kamen Rider Build.
Sento Kiryu starts his new job at the Touto Institute of Advanced Matter Physics, researching the mysterious Pandora's Box. While responding to a potential Smash sighting, Sento comes across a wanted fugitive...
The Kamen Rider of Touto is now a fugitive, due to his part in saving a convicted murderer - Ryuga Banjo. Kiryu begins to see similarities in the mysterious experiments both he and Ryuga were subjected to.
The mysteries of the past continue to pile up as Sento investigates the Pandora's Box disaster. In Seito, Ryuga and Sawa continues the search for Nabeshima's family.
A debate about the nature of Katsuragi's research leads to another falling out between Sento and Ryuga. Under pressure from a backer, Faust hatches a plan to steal Pandora's Box.
With Pandora's Box safely in their possession, Sento and Ryuga go after Gentoku, set on revealing the link between Touto's government and Faust to the public.
Hokuto invades Touto, looking to claim Pandora's Box for themselves. As civilian casualty numbers raise, Gentoku's government instigates Kamen Rider as their military weapon.
Another clash between the Touto and Hokuto Riders ends with Ryuga struggling from the Sclash Driver's effects. The Hokuto Crows are given a way to power up by Blood Stark, but there's a cost...
Soichi gives Sento the means the stop Ryuga's growing rage with the forbidden item - the Devil's Trigger. Gentoku plots a counter-attack against Hokuto's invaders.
Touto and Hokuto's governments have ordered a winner-takes-all battle to decide who keeps Pandora's Box. It's Build vs Grease to decide the victor of the Rider War!
Seito delivers an ultimatum to Kazumi - fight for them as their Kamen Rider or witness the death of his family. A mysterious new adversary makes his way to Touto to confront Sento and Ryuga.
Kamen Rider Rogue receives the data on Build's Rabbit Rabbit form, forcing Sento to unleash Tank Tank form; Sawa's backstory is revealed while Kazumi rescues civilians; Nanba decides to not play fair, taking control of Seito and declaring war.
Despite their promises, Seito continues their assault against Touto. All this is made worse as Blood Stark completes a panel of Pandora's Box, and the Skywall begins to shift...
Sawa and Misora discuss what the story behind Ryuuga really is; Team Build raid Pandora's Tower in an attempt to reclaim Pandora's Box, but Blood Stark stays one step ahead, goading Ryuuga into action.
Pandora's Tower is complete and Evolt stands ready to destroy Earth. Kazumi is still angry with Gentoku for killing his friend and challenges him to a fight; Sento works on a way to harness Pandora's Box into a power-up for Build.
In order to keep Evolt from getting Pandora's Box, Bernage moves the entire cafe to a different location; Ryuuga can no longer transform and so he and Kazumi go to Gentoku for help in saving Sento.
Kazumi is bummed that Evolt has stolen the bottles of his lost friends. Sento travels to his mother's house in Hokuto for answers about his father, but he and Kazumi end up face to face with a new Smash.
Sento learns that his father actually is alive; Sawa goes to find the one Faust experiment that, to their knowledge, has no relation to the Kamen Riders; Misora discovers that the Best Matches are about her.
Kazumi uses his new weapon to battle the clones of his former friends; Gentoku does battle with another set of clones and gets help from an unlikely ally.
