The film begins with Sento having a dream of the moment when Kamen Rider Build extracted Kamen Rider Ex-Aid’s essence, an event he barely remembers, while Misora creates the Doctor and Gamer FullBottles from it. Soon after, Misora informs Sento and Ryūga of some Smash activity and they confront the enemies, who are in fact Bugsters, who are immune to their attacks, until Sento uses the new bottles to transform into Ex-Aid himself. However, a mysterious attacker called Left Kaiser appears and steals the bottles. Ryūga chases after him, just to be thrown into a machine that sends him to a parallel world where the Skywall does not exist, while Sento retrieves the bottles, but Left Kaiser flees without telling him what happened to his friend. Suddenly, Sento’s world shifts between dimensions and appears next to the other world, spreading hordes of Bugsters in both. With Emu unable to transform into Ex-Aid, his fellow Riders take the initiative against the Bugsters, until another enemy similar with the one who appeared in Sento’s world, called Right Kaiser appears to stop them, revealing that the Bugsters he is spreading are fuel for his machine, Enigma, which is similar to the one Left Kaiser has. When both Enigmas activate, their energy seals the powers of the Gamer Drivers and Gashats. Right Kaiser then attempts to kill Emu and co., but Kamen Rider Ghost appears and protects them, and he leaves after revealing that the world will be destroyed in 24 hours.Meanwhile, Sento is attacked by Parad, who affirms that he spent the last two years looking for Build in his world to get back Ex-Aid’s essence. Their fight is stopped by Sōichi, who reveals that the culprit must be Kaisei Mogami, a researcher who once published a paper about the development of Enigma, a machine for contacting parallel worlds. Using the Gamer FullBottle, Parad manages to contact Emu and inform him about the situation as well. Sōichi then gives the Phoenix and Robot FullBottles to Sento before leaving. Kuroto, having developed a new Gashat with data from his previous fight against Build to briefly regain his powers, is convinced by Onari to create more for the other Gamer Riders, while Sento and Sawa discover that Kaisei once worked under Takumi Katsuragi on the development of Enigma, and created a biological weapon called the “Kaiser System” using the Bugster Virus and the Nebula Gas to create “Nebula Bugsters”, as well as the Left and Right Kaisers. In the occasion, Sento concludes that it was Takumi who used the Build Driver to travel to Ex-Aid’s world and steal his essence, although he can’t figure why he has memories of it.As Sento and Parad fight the Nebula Bugsters to protect the population, Left Kaiser appears to stop them, but Sento drives him away with the Phoenix and Robo FullBottles. The two then track down his hideout with Misora’s help and Sento concludes that Takumi stole Ex-Aid’s powers to prevent them from being sealed as well, as a countermeasure against Enigma. With the new Kamen Rider Build Gashat created by Kuroto, Taiga, Hiiro and Kiriya join Takeru against the Nebula Bugsters, while a cliff suddenly opens and Ryūga is about to fall on it, when Eiji Hino appears and assists Emu in rescuing him. Eiji then reveals that the Foundation X is working with the Kaisei Mogami’s incarnation in their world to develop their side’s Enigma and the three set out to confront him at his base, while Sento confronts the other Mogami, who reveals that he suffered an accident that transformed parts of his body after Takumi attempted to stop him from completing Enigma. He and his counterpart then reveal that their true objective is to use Enigma to combine both worlds, while fusing their bodies into a single, immortal entity, called Bi-Kaiser.To fight Eiji and the others, the Mogami from their side uses some Core Medals developed by Foundation X to create copies of several Greed, including Ankh, to fight them. However, Eiji takes the opportunity to fuse the original Ankh’s broken medal into the fake Ankh’s body to revive him, also regaining the power to transform into Kamen Rider OOO, while Kamen Rider Fourze appears to save Emu and Ryūga, and Kamen Rider Gaim appears to assist Sento and Parad. With help from Fourze and Gaim, Sento and Emu reach the Enigma machines on their respective worlds and Sento concludes that the best way to solve the situation is by activating them before they reach their full potential, which they do, causing the two Mogamis to fuse into Bi-Kaiser as expected, but with his powers incomplete, thus not becoming invincible. Taking advantage of this, Sento returns Emu’s powers and while the other Riders face Bi-Kaiser’s forces, Build and Ex-Aid confront and destroy him along with the Enigma machines, stopping the fusing of the worlds. Sento and Ryūga then bid farewell to Emu, Parad and the other Riders, who return to their world before the worlds get separated again. After the battle, Onari is welcomed back in the Tenkuji temple, while Ankh’s temporary body fades away, with Eiji promising that they will meet again.In the post credits, Sento and Ryūga confront several Guardians from Faust when a mysterious Rider appears and steals the Phoenix and Robot Bottles from Sento, as he flees, he claims that the Bottles belonged to Hokuto in the first place, and affirms that he will soon return.Overall, Kamen Rider Heisei Generations Final – Build, Ex-Aid and Legend Riders is a fitting send off for this series of crossover films, and does its best to tie missing elements of the Build mythos together on the way. Seeing previous riders return and combining elements of those series with Build and Ex-Aid makes for a fun ride, and I highly recommend that any longtime Kamen Rider fans check out this film, if for no other reason than to see how the different main series Riders interact.