While walking back home with his friends, Sougo Tokiwa is ambushed by Geiz's Time Mazine, however the Another Den-Liner appears from the time continuum and makes a crash landing. The Super Time Jacker Tid walks out of the Another Den-Liner and proclaimed that he would destroy the legacy of the Heisei Kamen Riders. Another Den-O jumps out and attacks Sougo, who has apparently lost his memories of being Kamen Rider Zi-O. After touching his dropped Zi-O Ridewatch, Sougo's memories return, and both him and Geiz transform to fight Another Den-O, but it escapes. After canceling their transformations, the two are met with an audience who applaud them. Tsukuyomi, who's memories had just been altered, then yells at the two by their last names, saying they were late for school.
Meanwhile in the city, a boy named Shingo is caught in a green tornado and falls into the arms of Ryuga Banjo. While Sento Kiryu attempts to make a scientific guess as to what just happened, they are met with Another Double, who attacks the now-transformed Build and Cross-Z. While Ataru Hisanaga happily records the fight, Kazumi Sawatari as Grease and Gentoku Himuro as Rogue arrive to help Build and Cross-Z fight Another Double. After they fend him off, Sento and Ryuga are then confused as to why Kazumi and Gentoku have their memories, as they did not cross through the new world. Later when they realize the boy from earlier was gone, Sento sets out to find him. Later at that moment, Gentoku loses his memories and walks off. Kazumi and Ryuga walk the city trying to figure out what happened to Gentoku and the new world situation, when Misora Isurugi arrives and berates Ryuga for not calling or texting her, while ignoring Kazumi. Ryuga is surprised to see that Nascita has now changed to Nasubi, when Misora suddenly loses her memories and happily runs off from who she thinks are her "fans".
At Hikarigamori High School, Sougo and Geiz notice that Tsukuyomi's memories have been heavily altered, and her personality seems too "normal". Ataru approaches her and refers to her as "Tsukuyomi", which Geiz found weird as that is not a name that anyone in the current time period should know, except Sougo and Geiz. Sometime after school, Tsukuyomi and her friends decide to head to the formerly 9 5 DO, now Mojimoji to eat. Geiz follows them in and is greeted by Junichiro Tokiwa who offers to take his order. Geiz politely declines, but his poor choice of words landed him a cup of coffee.
Sento finds Shingo and reveals that he is Kamen Rider Build, however Tid appears wanting to take Shingo away. Sento enters a fight with him to protect Shingo, but he gets brainwashed himself and is forced to do Tid's bidding. Tid becomes unpleased that Riders exist in this world despite them being fictional and wants to know why. He reveals that Shingo is the key to his plans. In secret, Sento programs his Build Phone.
Ataru walks home and discovers that Sougo was following him, so Ataru introduces himself as Sougo's classmate. He also reveals that he has the ability to draw Kamen Riders to him, so he knows Sougo is Zi-O. Sougo didn't seem convinced, so Ataru takes him to Futo City to eat at Master of Fumen's shop, hoping to draw out Kamen Rider Double, however they end up drawing out Another Double instead, who was chasing Shingo. Sougo attempts to transform, but Another Double escapes at the sight of seeing Sougo's Driver and Ridewatch. Master then discovers that Sougo was the one that Shotaro Hidari was talking about, so he presents him with the Double Ridewatch. Sougo then talks to Shingo and offers to help him find his mother, however Shingo replies that his mother is currently in the hospital giving birth to his younger brother. He also expresses interest in watching Kamen Rider Kuuga, which Ataru is surprised to hear such a young kid be interested in the classics. Ataru reveals he was born the day before Kuuga had aired its first episode on television, and Shingo thought today was Ataru's birthday.
Sougo brings Shingo and Ataru to the now Cafe Mojimoji, where he finds Geiz studying and has now lost his memories. Outside, Sougo tells Shingo that he should bring him home and brings out his map app on his smartphone, which Shingo replies that it was his first time seeing a "small laptop". Ryuga and Kazumi arrive and see Shingo, who he reveals that after Sento found him, he was caught. Sento's Machine Builder arrives unmanned, which Ryuga deducts that Sento sent it here so they can find him.
Ataru, Kazumi, and Shingo ride the bus home. Ataru finds that Kazumi highly resembles Otoya Kurenai from Kamen Rider Kiva (a reference that both Kazumi and Otoya are portrayed by Kouhei Takeda), but Kazumi replies in confusion. The three get off and arrive at Ataru's house. Discovering that the nameplate is "Hisanaga", Shingo asks for Ataru's name and it strikes familiarity with him. Another Double appears and Kazumi transforms to fight him. Another Den-O arrives and helps Another Double gang up on Grease, and they defeat him. Another Double approaches Ataru and Shingo, but Shingo assures to his younger brother that everything would be ok. He jumps in front and shields Ataru, but is then taken by Another Double. Another Den-O walks up to Ataru and reveals his identity to be Ataru himself, from the future.
Taking the bike, Cross-Z and Zi-O arrive and Tid and Sento's location. Sento transforms into TankTank form and fights the two. Build manages to take down both Cross-Z and Zi-O, and prepares to finish them. However at the very last moment, he turns the blaster and fires at Tid instead, who then uses his powers to stop the attack. Realizing that Sento was only feigning his brainwashing, he did not care as Another Double had already completed his mission by kidnapping Shingo, and leaves. Sento then apologizes to Ryuga and Sougo, as he had to trick his allies first before he is able to effectively fool the enemy. Sento then realizes that the entire fight was actually a diversion in the first place, to lure them away from Shingo and Another Double.
Sougo and Sento then return to Ataru's house looking for "Shingo", but Ataru doesn't know why the two would know the name of his older brother, and realizes they were referring to the boy. He brings them to his room which is filled with Kamen Rider memorabilia, with Sougo and Sento stunned to see toys of themselves. Ataru reveals that Kamen Riders are fictional and were brought to this world due to him making a contract with an Imagin named Futaros. He also reveals that on the day he was born, his older brother of seven years, Shingo Hisanaga went missing. As he grew up a fan of Kamen Rider, he wished that they would one day come to life and help find his brother, which was the main conviction of his contract with Futaros. Realizing that it would never happen because they were fictional, Ataru heeded no mind to the situation with Tid and the Another Riders. This angered Sento, who berated Ataru for having no compassion for people, real or not, and walked away. As the two walk downstairs, Sento discovers a picture. Futaros then reveals to Ataru that his powers are diminishing, and the remaining Riders left in the world will soon disappear.
At Tid's newly created Another Double/Den-O tower, he meets up with the two Another Riders who have successfully taken Shingo. He puts Shingo inside a box, and declares his intent to correct the world. The tower then rises to its apex and causes the weather to change to heavy rain. Sougo began to have an existential crisis, but Sento reassures him that as a guy who used to not exist, he had learned to deal with it and convinced Sougo to as well. The two transform in the rain and fight the horde of soldiers that stand in their way. Inspired by what Sento said, Ataru walks out of his house and finds Kazumi, but he has lost his memories and cannot help him. Ataru is downed by a group of soldiers, but is saved by Futaros. Finishing up his contract, Futaros enters Shingo. At that moment, Sougo arrives and sees Futaros enter. Because Ataru has lost his conviction, Sougo, Sento, and Ryuga disappear.
Sougo then awakens in 9 5 DO and explains what happened to Black Woz. Sento then comes into the shop, and tells Sougo to recall the photo they saw at Ataru's house. The photo is revealed to be a picture of Shingo, which Sougo deduced that Shingo is in fact Ataru's older brother from the past. Knowing what to do, he reveals the Double Ridewatch and uses it to transport the three back to Ataru's world. Sougo arrives at where Ataru was at last, and Woz uses the Gaia Library to ascertain the date that Futaros has gone back to, January 29, 2000, the day Ataru was born. Sougo takes Ataru to that date, to the hospital where Futaros had just arrived and begun to chase after Another Double. Another Double takes "Shingo Hisanaga" hostage. After hearing Shingo's last name, Sougo reveals Shingo's true identity to Ataru. After realizing that his older brother was trying to protect him after all this time, Ataru lashes out at Another Double, but is forcibly transformed into Another Den-O. Another Den-O then leaves with Another Double and Shingo aboard the Another Den-Liner and travel back to where they were at the start of the film. Sougo reveals that he already notified his friends to meet him.
At the mummified tomb of Riku, the original Kamen Rider Kuuga, Tid steals its powers into an Another Ridewatch. Geiz arrives in an attempt to stop him from destroying the Heisei Rider legacy. Tid turns himself into Another Kuuga, and Tsukuyomi fights him using his Time Mazine, which unfortunately gets trashed. Sougo and Futaros follow through using Sougo's Time Mazine, but they are ambushed by Another Kuuga and the Another Den-Liner, which ends up trashing up their Time Mazine, and they crash into the mountains. Tid turns back into his human form, and the Another Den-Liner travels foward to December 3, 2018 at the start of the film. As it crashes into Geiz's Time Mazine, Shingo takes this chance to run, though Another Double catches him to him and chases him to the city.
By making himself Another Kuuga, Futaros deduces that Tid already erased the history of the Heisei Riders, and reveals that Shingo is an Singularity Point, meaning temporal changes in his past will not affect him. Futaros announces his intent to help save Shingo and the world, but is distraught that Sougo's Time Mazine is wreaked. After Geiz and Tsukuyomi arrive, they attempt to travel back to 2018 together, but their temporal system were too damaged to get them anywhere. Miraculously, they are saved by the real DenLiner.
Back in present day, Sento and Ryuga confront Tid, who is angry that the Riders still exist. The two transform into RabbitRabbit and Cross-Z Magma and fight the soldiers. After taking the fight outside, Sougo and his team return with the DenLiner's help, and Sento welcomes them back. Sougo, Geiz, and Futaros confront Tid and the Another Riders, who voices his intent on erasing the Heisei Rider legacy. Sougo and Geiz attempt to transform, but Tid stops them and ambushes soldiers on them, while Futaros attemps to save Ataru from Another Den-O. Before Another Den-O is able to finish Futaros off, everyone in the room (except Tid) are transported to safety by Kamen Rider Den-O, which Momotaros, Kintaros, Ryutaros, and Urataros take turns fighting the soldiers and defeating Another Den-O, turning Ataru back to normal. Urataros as Ryotaro Nogami helps him up and reveals that Kamen Riders do and will continue to exist as long as people remember them. They then transport the team back, and the Taros fight amongst themselves. While Ryotaro is seemingly possessed by Urataros, Momotaros revealed that he wasn't fooled and Ryotaro smiles.
The four return to the tower, and Ataru tells Tid what Ryotaro had said to him. With his conviction now at its maximum, all the Riders that had lost their memories return, including Kazumi and Gentoku who transform into Grease and Rogue. Two children are ambushed by a pair of soldiers, but their cry for help summons Kamen Rider Ex-Aid and Kamen Rider Ghost to their rescue. As two young men are about to be attacked, Kamen Rider Double appears to save them. A young man encourages two boys to believe in the Kamen Riders, which results in him summoning Kamen Rider Kuuga. In the public, Kamen Rider Gaim and Kamen Rider Kiva appear, followed by Kamen Rider Wizard, Kamen Rider Ryuki, and Kamen Rider Agito. Kamen Rider Blade, Kamen Rider Faiz, and Kamen Rider Hibiki then follow through. Kamen Rider Kabuto, Kamen Rider Fourze, and Kamen Rider Decade arrive as well. Last but not least, Kamen Rider Drive and Kamen Rider OOO appear. Momotaros takes over as Den-O Sword Form and makes his stand against a group of soldiers.
While Another Double fights Build and Cross-Z, Sougo arrives and transforms into Zi-O DoubleArmor, with Woz giving a speech shortly after commemorating DoubleArmor's first appearance. With the new Armor, Zi-O defeats Another Double. An enraged Tid transforms into Another Kuuga and attacks Zi-O and Geiz. They take their Time Mazines, now fixed by Sento to fight Another Kuuga. Geiz gives Sougo the Kuuga Ridewatch, which he had created at Riku's gravesite before Tid forced him out. After being defeated, Zi-O and Build chase Tid back to the tower, and he merges himself with Shingo before transforming into the giant Another Ultimate Kuuga. The two follow him on their bikes to an empty battlefield, with the public cheering them on. However Tid summoned a horde of soldiers, which then all the Riders appear to help the two clear the battlefield. The Riders then charge their final Rider Kicks alongside Kuuga himself in order to defeat Tid, and save Shingo. In the midst of the battle, Shingo wakes up and is met with the sight of all 20 Heisei Riders assembled.
As Shingo prepares to be taken back to 2000, Ataru thanked his brother for protecting him, and Shingo apologized for making his younger brother wait to see him after 18 years. Futaros considered his job complete and walks away. The DenLiner departs and the timeline changes. On January 29, 2000, Ataru Hisanaga is born with Shingo and a Kuuga figurine and plush right beside him. In 2003, the two pose with Kamen Rider Faiz gear. In 2005, the two pose with Hibiki gear. In 2007, the two attend a Kamen Rider Den-O show, posing with Den-O himself. In 2010, the two pose with the Cyclone and Joker Gaia Memories. In 2018, the two pose with Zi-O memorabilia.
In the new timeline, Sougo walks home from school and meets up with Sento, who both found themselves to still exist in the world. Sento decides to let this matter slide and just continue to exist, and Sougo proudly agreed. During the end credits, Sento and Ryuga ride off on their bike, and Sougo meets up with Geiz and Tsukuyomi.
