The film opens with Zawame City torn apart by a brutal clash between the Heisei Riders and the Shōwa Riders, a cataclysmic showdown that shakes the present and sends the audience back to the week prior. As rumors swirl about a mysterious underground force, Kota Kazuraba [Gaku Sano] investigates the area with his friend Mai Takatsukasa [Yuumi Shida], only to tumble into a hidden chasm that drops them into an alternate, underground version of their city. There they meet a boy named Shu [Shun'ya Shiraishi], who possesses the uncanny ability to revert objects and events, yet cannot recall why his power matters.
Kota and Mai drag Shu back to the surface, where Takeshi Hongo [Hiroshi Fujioka] makes a sudden appearance, demanding the boy be handed over. The relief is short-lived, as Harbingers from the Underground Badan Empire descend and kidnap Shu themselves. In the ensuing chaos, Takeshi transforms into Kamen Rider One to enable Kota, Mai, and Shu to escape. The surface erupts in battle as Armored Riders Baron, Ryugen, and Zangetsu Shin join the fray, with Kota stepping forward as Armored Rider Gaim to shield his friends. Yet Gaim’s advance is halted when Kamen Rider Fifteen arrives, wielding a Lockseed that unlocks the powers of the fifteen Heisei Riders, forcing a confrontation as One, Two, and V3 appear to intervene. In a desperate bid to flee, Shu uses his powers and escapes with Gaim and Mai.
The trio seeks refuge at Drupers, where Tsukasa Kadoya and Narutaki appear, urging that all fifteen Heisei Riders unite to halt Badan’s plans. Narutaki warns that begging for Shōwa Rider aid would be futile. Tsukasa embarks on a mission to rally the other riders alongside Kaito Kumon, while Kota stays behind to care for Shu. The first contact is Shotaro Hidari, who refuses to help until he finishes a lingering job—tracking down some lost pets. Tsukasa leaves Kaito to assist Shotaro and reaches out to Takumi Inui, who likewise refuses, insisting his days as a Rider are over. Kamen Rider Fifteen materializes and assaults them, prompting Tsukasa to transform into Kamen Rider Decade to fend him off, allowing Takumi and Tsukasa to slip away.
In a ramen shop, Takumi intervenes when a wounded runaway holds Mari hostage. Keisuke Jin arrives to heal the wounded man and convinces him to surrender to the police, after which Takumi accompanies Jin to his clinic. There, Jin tends to another patient and helps Takumi confront the lingering guilt over Masato Kusaka’s death, encouraging him to stay at the clinic for a while.
The next day, Badan’s forces strike again. To protect Mari, Takumi reenters the field as Faiz and defeats the attackers, but Jin reveals himself as X and claims Takumi must find resolve on his own. Meanwhile, Tsukasa finally persuades Shotaro to hear him out and asks about Shu. In a poignant turn, Shu finally remembers his past and leads Kota to his home, where it’s revealed that Shu has been dead all along. Before Shu’s death, he argued with his mother and now longs to see her one last time. Kamen Rider Fifteen reappears to take Shu away, and Kota learns that Ren Aoi, Shu’s father, is the Rider’s true identity. Ren reveals that Badan intends to power a machine called the Mega Reverse, which can resurrect the dead at the expense of the living.
As the two worlds collide, Kamen Riders Gaim and Decade duel Fifteen while, elsewhere, the Shōwa Riders converge with the Heisei Riders. Decade rescues Shu and ferries him toward his mother, yet General Jark blocks the way. The Shōwa Riders, assisted by Black and Black RX, are temporarily repelled by Shotaro’s Joker, allowing Tsukasa to continue escorting Shu. Kamen Rider Baron lends support to the Joker squad as Tsukasa guides Shu to a lighthouse where his mother awaits. But Fifteen closes in, devastating the standstill duo with the power of the Heisei Rider Lockseed.
Back in the present, the Shōwa and Heisei Riders clash as the Shadow Line’s General Schwarz aligns with Badan and attacks Kamen Rider Den-O’s Den Liner. Kota, Tsukasa, and Takumi join forces with the remaining Heisei Riders to confront the Shōwa Riders and understand their motives. One explains that the Heisei Riders’ attachments to their deceased loved ones fed Badan’s rise and the appearance of Fifteen. Ambassador Darkness, the Badan leader, is revealed to be Ryo Murasame, who infiltrated the group to sabotage it from within. He betrays the organization, but the Mega Reverse Machine is revealed to be Badan’s base, which lifts into the sky.
Ryo summons the other Rider allies back from Helheim Forest to support him and Gaim, and the Heisei-Shōwa alliance presses on. The combined forces face a colossal threat: a giant Badan Generalissimo monster. Daigo Kiryu of the Kyoryugers arrives with Gabutyra, but the beast proves formidable. The ToQgers and Kamen Rider Den-O arrive, combining their trains with Gabutyra to form ToQ-Oh Kyoryuzin feat. DenLiner and destroying the Generalissimo. As Decade rescues Shu from the Mega Reverse Machine, Haruto Soma [Shun'ya Shiraishi] and Jin prepare to destroy the machine for good, but Takumi seeks a chance to revive Masato. The three Riders persuade each other that destroying the Mega Reverse is the correct course, and together they shatter it as Wizard, X, and Faiz.
In the climactic moment, One bestows upon Gaim a special Lockseed containing the powers of the fifteen Shōwa Riders, enabling him to transform into One Arms and decisively defeat Fifteen, who regains his senses. With the Badan Empire nearly defeated, the Shōwa Riders assist in reuniting Shu with his parents one final time before sending him on. They then bid farewell to the Heisei Riders, prepared to return to their own realm, leaving the Heisei Riders to wonder if they can protect Earth without their Shōwa counterparts. The decisive act comes when Gaim, sacrificing nothing, shields a lone flower from One’s Rider Kick, prompting One to acknowledge the virtue in such kindness and prompting a heartfelt reconciliation between the two worlds as they part ways.
An alternate ending offers a different sacrifice: Gaim contemplates a powered-up strike against One, but halts, recognizing the risk of a preemptive attack. One commends this restraint, and the Shōwa and Heisei Riders find their paths toward reconciliation as well.
