10 years ago when the Pandora Box was discovered on Mars and brought to Earth, it's activated and creates a Skywall which divides Japan into Touto, Seito, and Hokuto resulting into untold chaos. Along with creating a complex power dynamic between the three regions, the event brings waves of monsters called Smash down on the country, who terrorize and attack the citizens. Sento Kiryu, an amnesiac taken in by a mysterious cafe owner named Soichi Isurugi who provides him with a belt called the Build Driver that allows him to turn into Kamen Rider Build, which he uses to fight the Smash on his own.
Episode 1
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.
Episode 2
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.
Episode 3
Sento follows the clues regarding Nabeshima, the man responsible for framing Ryuga.
Episode 4
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.
Episode 5
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.
Episode 6
Ryuga deceives Sento, and tries to find Faust's hideout by himself.
Episode 7
There is a possibility that true culprit who killed Katsuragi was Sento, but Sento have lost his memory. Ryuga feels a distrust for Sento.
Episode 8
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.
Episode 9
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.
Episode 10
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.
Episode 11
If Blood Stalk's identity is Katsuragi, Ryuga can prove his innocence. But Sento can't control Key-Dragon form's power.
Episode 12
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.
Episode 13
Sento gets a terrible shock, because he learns the identity of the wirepuller from Nabeshima who regained his memory.
Episode 14
An expected person steals Pandora Box, Panels and Bottles from Sento.
Episode 15
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.
Episode 16
It's revealed the real facts of the case that Katsuragi was murdered.
Episode 17
As the three countries start a war, both Sento and Ryuga must decide what to do next.
Episode 18
Sento confronts Kazumi Sawatari, an assassin from Hokuto, wondering how the enemy got his hands on his latest invention.
Episode 19
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.
Episode 20
Soichi approaches Sento and gives him an item that can stop Ryuga's rampage, but using it involves a huge risk.
Episode 21
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.
Episode 22
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.
Episode 23
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.
Episode 24
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.
Episode 25
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.
Episode 26
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.
Episode 27
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.
Episode 28
Despite using his new powers, Sento is having a hard time against Gentoku, and Sawa reveals to the others the reasons for her betrayal.
Episode 29
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.
Episode 30
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.
Episode 31
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.
Episode 32
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.
Episode 33
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.
Episode 34
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.
Episode 35
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.
Episode 36
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.
Episode 37
Evolto avoided his destruction by possessing Sento's body. Now unable to transform, Ryuga looks for a way to save his friend.
Episode 38
Evolto reaches his full potential and seems unstoppable, while Sento recovers his memories as Katsuragi.
Episode 39
To face the full power of Evolto and Utsumi, Sento combines the power of all Full Bottles to unleash his most powerful form.
Episode 40
Surprised with the advent of Build's new form, Evolto decides to begin the last steps of his master plan.
Episode 41
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.
Episode 42
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.
Episode 43
As Sento still tries to figure out his father's true motives, Misora is kidnapped by Evolto and transformed into a Smash.
Episode 44
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.
Episode 45
Evolto has been defeated, and Shinobu reveals to the Riders his true objective.
Episode 46
With Evolto one step closer from reaching his objective, Sento and the others prepare themselves for the final battle.
Episode 47
Despite Sento's warnings, Kazumi uses the Blizzard Knuckle to attain a new, powerful form, knowing that he may not survive the transformation.
Episode 48
With no time to mourn for Kazumi's death, Sento, Ryuga and Gentoku brace themselves for the final battle against a full-powered Evolto.
Episode 49
After the sacrifice of his companions, it falls to Sento to protect the world by defeating Evolto once and for all.
