After the civil war’s end, Japan’s new city-state governors promise a new era of prosperity on inauguration day. The Touto governor Kengo Inō Masanobu Katsumura rouses anti-Kamen Rider sentiment, persuading the public to view Sento Kiryū Atsuhiro Inukai as an enemy of the state. The mob chases Sento as Kamen Rider Build after he saves a boy and his older sister at a stadium, while Kazumi Sawatari Kouhei Takeda and Gentoku Himuro Kensei Mikami cover his escape before their capture by Hokuto governor Ryōka Saiga Rena Matsui as the Scissors Lost Smash. After subduing Misora and Sawa once reaching Nascita, Sento attempts to call Ryūga Banjō Eiji Akaso who was abducted by Mitsuomi Gōbara Takashi Fujii, provoking him into a fight after transforming into the Zebra Lost Smash while revealing himself to be responsible for Kasumi Ogura’s death. The fight ends when Inō arrives, with Gōbara infecting a distracted Ryūga.
Sento hacks into a government facility’s database to confirm the new governors as Soichi Isurugi’s Kiwami Project teammates before being confronted by Inō. Soichi Isurugi Yasuyuki Maekawa is revealed as part of the conspiracy, and Evolto Tetsuo Kanao is described as a key figure among the Blood Race who served as a co-conspirator in manipulating events toward Earth’s destruction. But Evolto’s deviation from their plan forces the governors’ hand with their Build Annihilation Plan, as a brainwashed Ryūga attacks Sento as Cross-Z. Build falls, and Inō proclaims that the public now sees Kamen Riders as reminders of the civil war’s horrors. Ryūga then hands Sento’s Hazard Trigger to Inō, who uses it with his Cobra Lost Fullbottle and Ryūga’s Great Cross-Z Dragon to become Kamen Rider Blood while absorbing his kin, as Evolto carries the unconscious Sento away.
Safe at last, Sento wakes from a dream of Takumi Katsuragi’s memories of the Skywall Disaster, where his father Shinobu Katsuragi [Jouji Kokubo] tells him that Ryūga is their ace against the aliens. Evolto belittles Sento while explaining that Inō only needs the Pandora Box to destroy the world, and a despondent Sento walks off until Inō calls to trade Ryūga for the Pandora Box. Sento engages Takumi in a metaphysical debate, his past self lamenting failing to honor his father’s request as the aliens perverted his research for their schemes; Sento counters that Project Build allowed them to protect their friends and insists that Ryūga is essential to Evolto’s defeat.
The next day, Sento brings the Pandora Box to the governors with Inō going back on his word and challenging Sento to take Ryūga back by force. Sento accepts, transforming into Build Genius, and remains steadfast in a losing battle before using the Gold Rabbit Fullbottle’s power to extract Ryūga from Blood alongside Blood’s kin. Blood, however, remains defiant and merges the Hazard Trigger with the Pandora Box to threaten the planet’s destruction. Kazumi Sawatari [Kouhei Takeda] and Gentoku Himuro [Kensei Mikami] join Blood’s kin in battle after Nariaki Utsumi [Yuki Ochi] eventually frees them, and Sento notices Ryūga’s Silver Dragon Fullbottle, realizing their Fullbottles together could destroy Evolto and his kin. Vernage intervenes to restore Misora and Sawa before merging the two Fullbottles with the Genius Fullbottle into the Cross-ZBuild Can, with Sento unintentionally merging himself and Ryūga into Kamen Rider Cross-ZBuild.
Cross-ZBuild pushes Blood back to the surface while Kamen Riders Grease and Rouge destroy the Zebra and Scissors Lost Smash, underscoring that Riders’ drive to build the future persists even as Evolto gloats over his treacherous subordinates’ downfall. After the Cross-ZBuild Can is defused back into its components, the group remains wary of continued anti-Kamen Rider sentiment, though Sento stays unfazed as long as those who can build the future remain protected, especially after the boy he saved previously thanks him.
Sometime later, Sento studies how to reproduce a Cross-ZBuild-like fusion to defeat Evolto, only to be teleported by a white Pandora panel into a battle among various Kamen Riders and their enemies. The monsters are then wiped out by a mysterious Kamen Rider donning Build Armor, who introduces himself as the apparent demon king Kamen Rider Zi-O, played by So Okuno.
