The Hikari Studio arrives in a new AR World, and, as Tsukasa Kadoya, Masahiro Inoue's photos of this realm come through with striking clarity, Natsumi realizes they may be stepping into Tsukasa's home world. Tsukasa remembers a mansion shown on a backdrop that brought them here, and when they locate the manor, he opens the door. There they meet a girl named Sayo who identifies Tsukasa as her older brother. Sayo's caretaker Nobuhiko Tsukikage arrives and confronts him about leaving his sister alone for so long. But as the Nine Worlds fusion begins to influence the World of Decade, Tsukikage reveals that Tsukasa's journey has now unlocked a potential remedy to the crisis: the Rider Battle, a contest designed to determine the strongest Kamen Rider in existence. When Tsukasa’s memories return, he gives Tsukikage the green light to make it happen.
Three days pass, and after a string of intense matches, the showdown narrows to a six-man tag team clash: the team of Decade, Kuuga, and Diend versus V3, BLACK, and Super-1. With Diend briefly fleeing and Kuuga wounded, Decade must carry the weight alone and topple the trio by himself. His victory triggers a startling turn of events as a castle rises from the stadium, thrusting Yusuke and Natsumi into a throne room where they are welcomed by Dai-Shocker. Doctor Shinigami then reveals himself, toasting to Tsukasa’s “return” and exposing the truth that all the prior battles were engineered to secure Dai-Shocker’s dominion across the saved worlds. Tsukasa sends the frightened Natsumi and Yusuke into the sewers, attempting to shield them from the looming threat.
Back at the Kadoya mansion, the past begins to surface. Sayo shares insights about Tsukasa’s childhood, and the twins discover that she and Tsukikage have aligned with Dai-Shocker. The Earth Stone is used on Yusuke, unlocking Kuuga’s most dangerous power within him. Yusuke pleads with Natsumi to run as the unleashed force surges through him. In the enemy ranks, Dai-Shocker’s castle hosts a massive confrontation with the heroes, while the remaining worlds continue to converge. Tsukasa learns from Tsukikage that the Battle Fight was a ruse, and Sayo soon declares herself High Priestess Bishium as Tsukikage admits he has outlived his purpose and sinks into the sewer. There, Yusuke, now in Kuuga Rising Ultimate Form, battles Shadow Moon’s control.
Seeking a path to redemption, Tsukasa heads back to the Hikari Studio to try to reconcile with Natsumi, but she refuses to forgive him and chooses to carve out a new life, making the Studio her lone home. The next day, Dai-Shocker launches its multi-world assault, starting with the World of Decade. Narutaki discovers Natsumi in another conquered world just as Daiki, who has been disguising himself as a Dai-Shocker soldier, arrives. Narutaki senses a glimmer of hope: the possibility of rallying the Kamen Riders to counter the invasion.
Meanwhile, with few allies left, Tsukasa crosses paths with Jōji Yūki, a man hunting revenge for the loss of his arm while working for Dai-Shocker. When he realizes Tsukasa has changed and now stands in the crosshairs of their plans, Yūki rips away his own arm to reveal a robotic replacement and prepares to transform into Riderman. A separate engagement sees Diend and Natsumi ambushed by a Dai-Shocker platoon under General Jark. Diend calls on Raia, Gai, and Punch Hopper as a distraction while Tsukasa arrives, determined to confront Dai-Shocker and rescue his friends. Decade and Diend manage to repel Jark’s advance, and Jark is destroyed.
The battle escalates as all the Riders converge on Dai-Shocker’s castle. Doctor Shinigami and Ambassador Hell lead the army, with Ikadevil and Garagaranda in support. The tide finally turns when the other Riders arrive to bolster the defense; Garagaranda and Ikadevil are defeated, and the Bishium-controlled Rising Ultimate Kuuga appears with the aim of killing Decade. Sayo awakens to the consequences of her choices, destroys the Stone of the Earth, and frees Yusuke from its hold, while Shadow Moon moves in for the kill. In a dramatic turn, Kamen Rider W makes a striking entrance, taking down Shadow Moon and then slipping away, leaving the war to the others. With renewed unity, the Riders unleash an All Rider Kick to obliterate Shadow Moon and topple the Dai-Shocker castle.
The blast of power from that victory awakens King Dark in the wreckage. Diend arrives with Kamen Rider J, and, using the DecaDriver Final FormRide card, transforms Decade into Jumbo DecaDriver. J then becomes Decade Complete Form Jumbo Formation, delivering a Dimension Kick that destroys King Dark, using the other Riders as enormous Kamen Ride Cards. After this hard-won triumph, Tsukasa parts ways with the Riders but makes amends with Sayo as she begins a new journey of her own. Tsukasa returns to the Hikari Studio, his true home, and Eijiro reappears as if nothing happened, signaling a paradox resolved and a future that can still be shaped by memory, courage, and the bonds forged across worlds.
In the end, the narrative centers on memory, loyalty, and the desire to protect different realms from a shared threat. The world-trotting chaos tests every ally’s resolve, but it also clarifies who Tsukasa truly is: a keeper of connections across a multiverse of heroes, who finds a home not in the places he travels, but in the people he fights beside and in the studio where he belongs.
