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.
Sento follows the clues regarding Nabeshima, the man responsible for framing Ryuga.
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.
While confronting Souichi about his connection with Faust, Sento is informed by Sawa about a hint regarding a person who may know his true identity.
Ryuga deceives Sento, and tries to find Faust's hideout by himself.
There is a possibility that true culprit who killed Katsuragi was Sento, but Sento have lost his memory. Ryuga feels a distrust for Sento.
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.
Upon learning of Faust's plan to steal the Pandora Box, Sento and Ryuga set out to Touto Institute of Advanced Matter Physics to protect it.
If Blood Stalk's identity is Katsuragi, Ryuga can prove his innocence. But Sento can't control Key-Dragon form's power.
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 gets a terrible shock, because he learns the identity of the wirepuller from Nabeshima who regained his memory.
An expected person steals Pandora Box, Panels and Bottles from Sento.
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.
It's revealed the real facts of the case that Katsuragi was murdered.
As the three countries start a war, both Sento and Ryuga must decide what to do next.
Sento confronts Kazumi Sawatari, an assassin from Hokuto, wondering how the enemy got his hands on his latest invention.
With he and Ryuga unable to beat Kazumi and his men with their current strength, Sento searches the files left by Takumi to look for an alternative.
Soichi approaches Sento and gives him an item that can stop Ryuga's rampage, but using it involves a huge risk.
Despite knowing the risks, Sento uses the Hazard Trigger to empower himself against Kazumi's three subordinates. After losing his consciousness and killing one of them, Sento is struck with guilt and refuses to keep fighting, until Soichi gives him an alternative.
Sento and Kazumi have a duel to settle the war, but when the enemy proves himself too strong for him, Sento has no option but to use the Hazard Trigger again.
Taking advantage of the conflict, Seito invades and conquers Hokuto. Now bent on seizing all of Japan, the Seito forces invade Touto, led by the mysterious Kamen Rider Rogue.
Gentoku reappears as Kamen Rider Rogue, determined to get revenge on Touto. To stop him, the other Kamen Riders decide to join forces, when Sento perceives something strange happening with Misora.
Just as Gentoku is about to finish off Sento and the others, Misora appears before them, and the secret behind her mysterious powers is revealed.
Seito challenges Touto for another proxy battle to end the war once and for all. Sento and his friends make preparations to confront Gentoku and his subordinates, unaware that one among them is a traitor.
The proxy battle to end the war is tied with one victory to each side, and for the decisive bout against Gentoku, Sento unleashes a new power.
Despite using his new powers, Sento is having a hard time against Gentoku, and Sawa reveals to the others the reasons for her betrayal.
The war between the nations has ended, but Soichi and Namba finally reveal their true intentions as a new conflict begins when they open the Pandora Box.
Vernage, the Queen of Mars possesses the body of Misora to assist the Riders in retrieving the Pandora Box, revealing an important secret about Ryuga in the occasion.
As the Riders invade the Pandora Tower to retrieve the Pandora Box, a new power bursts from Ryuga, just as questions about his true identity are raised.
Doubtful about his own origins, and eager to know the reason why he was framed in the first place, Ryuga confronts Blood Stalk demanding for answers.
Touto's Prime Minister, Taizan Himuro is kidnapped, and Blood Stalk demands Sento to develop a new driver for him in exchange for his life.
Having attained the power to transform into Kamen Rider Evol, Evolto infects Sento with a poison, threatening his life should he not relinquish the Fullbottles in his possession.
Evolto is close to attain all Fullbottles and the Pandora Box, and with Ryuga under the enemy's control, Sento turns to Gentoku for help.
Having finally opened the Pandora Box, Evolto almost destroyed the world, should Vernage had not interfered. However, Sento has run out of options, and decides to risk it all in a move to defeat him.
Evolto avoided his destruction by possessing Sento's body. Now unable to transform, Ryuga looks for a way to save his friend.
Evolto reaches his full potential and seems unstoppable, while Sento recovers his memories as Katsuragi.
To face the full power of Evolto and Utsumi, Sento combines the power of all Full Bottles to unleash his most powerful form.
Surprised with the advent of Build's new form, Evolto decides to begin the last steps of his master plan.
Sento discover that his father is alive, and by searching the files he got from his mother, he learns why the Rabbit and Tank Full Bottles have a special meaning to him.
With Sento confused with the fact that his father might be in league with Evolto, another person transformed into Kamen Rider Build appears before him.
As Sento still tries to figure out his father's true motives, Misora is kidnapped by Evolto and transformed into a Smash.
Kazumi and Gentoku infiltrate Faust's headquarters in Hokuto, but Utsumi appears to face them. Meanwhile, Sento confronts his father, who reveals the true reason why he joined forces with Evolto.
Evolto has been defeated, and Shinobu reveals to the Riders his true objective.
With Evolto one step closer from reaching his objective, Sento and the others prepare themselves for the final battle.
Despite Sento's warnings, Kazumi uses the Blizzard Knuckle to attain a new, powerful form, knowing that he may not survive the transformation.
With no time to mourn for Kazumi's death, Sento, Ryuga and Gentoku brace themselves for the final battle against a full-powered Evolto.
After the sacrifice of his companions, it falls to Sento to protect the world by defeating Evolto once and for all.