- Written By: Kento ShimoyamaDirected By: Ryuta Tasaki
The year is 2068, and humanity is... not doing well. They're under the tyrannical reign of Oma Zi-O, a ruthless tyrannical world ruler possessing unrivalled, limitless power. All who oppose him are swept aside and obliterated, including soldiers from a resistance group. Sick and tired of this one-sided slaughter-fest, Geiz Myoukouin, a resistance fighter, goes back in time, hoping to kill the man behind Oma Zi-O's mask.
The year is 2018, and Sougo Tokiwa, a third year high-school student with a dream to become a king, goes to school where he is mocked by the others for having such a childish dream. One day, Sougo finds a strange object but wisely decided to not examine it. Back at home, his uncle Junichiro Tokiwa asks again about Sougo's seemingly unreachable dream of becoming a king in this age, abandoning even his future education.
Later, when Sougo is riding his bike, he meets a mysterious person who reads from a book about his future. Before Sougo can process his situation, a strange red robot ambushes him, but is whisked away by a black robot. This black robot is piloted by Tsukuyomi, a girl from the future who reveals that five decades later, Sougo becomes the infamous, ruthless Demon King called Oma Zi-O. A conflicted Sougo went out from the robot, only to get attacked by a Tyrannosaurus, in the Cretaceous era. Tsukuyomi saves him yet again, and drives them both to the Edo era.
Sougo asks Tsukuyomi again about his future, and she reveals that everyone in the future knows about him. She also warns Sougo about the strange object, a Ridewatch (that somehow is back on Sougo's possession), as its power is great and hazardous. Their discussion is interrupted by a fight nearby, and Tsukuyomi pushes Sougo to cease the conflict. However, Sougo is too timid to break the fight and he is thrown into a river for his troubles. As Tsukuyomi doubts Sougo being destined to become the tyrannical Oma Zi-O, the red robot, piloted by Geiz, attacks again.
Tsukuyomi reveals the red warrior to be Geiz, her friend from the resistance, who wants to kill Sougo before he becomes Oma Zi-O, but Tsukuyomi, wanting to settle things peacefully, speaks of her aims to make him a benevolent ruler instead. Geiz attacks Tsukuyomi's Time Mazine again, and in the confusion, Sougo goes overboard.
After coming to, Sougo sees a huge wall in the horizon. As he ponders, a strange lifeform attacks him, but two armored warriors came to save him. The two later reveals themselves as Sento Kiryu, Kamen Rider Build, and Ryuga Banjo, Kamen Rider Cross-Z. Sento took Sougo to his base of operations, the cafe nascita, causing Sougo to realize that he's now in the year 2017. Sento is surprised that Sougo came from the future, but before he asks anything, Tsukuyomi appears and knocks out the two Riders, as Sougo isn't supposed to interact with the past. Sougo wakes up back in his house and told his uncle about his adventures before, citing it all as a dream.
Back in 2017, at some part of the city, a basketball player is stuck in the path of an oncoming speeding car until a stranger saves him. That person is Heure, and he told the player that he's supposedly hit by the car today. Heure told the player that he can change his fate with his help, and the player agrees, accepting a menacing watch bearing a horrifying version of Kamen Rider Build's face on its display.
In 2018, the monster, using Build's ability, goes on a rampage, victimizing his former schoolmates. Hearing a ruckus, Sougo arrives at the scene, only for the monster to trounce him. Tsukuyomi arrives to fight the Build-like monster in his place instead, to prevent Sougo's rise to tyranny. Suddenly, the mysterious man with the book appears again, whom Tsukuyomi identifies as Woz, encouraging Sougo to fight and use his Ridewatch, but Tsukuyomi tries to dissuade Sougo.
Deciding to accept the seer's call, but considering Tsukuyomi's advice, Sougo resolves to be a great and kind overlord. Proud of his choice, Woz presents Sougo a Ziku-Driver and Sougo transforms to Kamen Rider Zi-O before defeating the monstrous Build with his newfound powers.
The battle is over, but the strife isn't finished yet, for Geiz appears yet again, having finally found Sougo and identified him. Using his own Ridewatch and the Ghost Ridewatch to gain armor for his Rider form, Geiz attacks Zi-O...
Episode Tropes:
- The Cameo: Sento and Ryuga appear in this episode and save Sougo from a Strong Smash Hazard when Tsukuyomi's Time Mazin is forced out of the time stream and fell into 2017.
- Deal with the Devil: Heure utilizes this trope to get a random person who was destined to suffer a Career-Ending Injury from a car accident to become Kaijin Rider Another Build.
- Future Me Scares Me: Sougo was really unnerved when he saw how dangerous and evil he would become in 2068, and had to leave the Time Mazin for some air. Near the end of the episode, he decides that if he becomes an overlord, then he'll be a good overlord.
- In Medias Res: The episode begins with the below mentioned battle with Oma Zi-O and an escape from a dinosaur that was salivating on Sougo, though it takes us until half the episode to get back to that point.
- Poor Communication Kills: At the end of the episode, Geiz questions why Sougo would willingly embrace becoming Zi-O and then tries to kill Sougo after hearing his answer, not knowing that Sougo plans to do what he can with his newfound power to help people. However, Sougo's specific phrasing of his answer did accidentally parallel his evil future self, which didn't help matters.
- Stealth Hi/Bye: Woz appears early into the episode, telling Sougo that his life was going to change from that point on, and then disappears without a trace after warning Sougo of the red robot.
- Temporal Paradox: Tsukuyomi attacked Sento, Ryuga, and Sougo in nascita specifically to try and prevent one from happening and limit the impact of them being in the past.
- Time Traveler's Dinosaur: When Tsukuyomi rescues Sougo in her Time Mazine, they end up in the Cretaceous period, where they encounter a Tyrannosaurus rex. Notably, most instances of time travel within the show are to and from times and places within the 21st century, making this unusual in comparison.
- The War Sequence: The episode begins with a charge of revolutionaries trying to stop Oma Zi-O with their own Time Mazin and soldiers. It should be obvious that they are thoroughly outclassed and couldn't even scratch him or the monument behind him.
- You Can't Fight Fate: Despite Tsukuyomi attempts to try and convince Sougo not to become Zi-O; he ultimately becomes Zi-O anyway.
