Upset, Shu sent a picture of himself holding a piece of paper reading 'I Hate You' before then being killed crossing the road. He remembers that before he died, his mother had been due to take him to see the lighthouse but was forced to cancel the plans when she had to work. Elsewhere, Shu finally remembers his past and leads Kouta to his house, where it is revealed that Shu has been dead the whole time. Meanwhile, Tsukasa finally convinces Shotaro to listen to his request, and asks him about Shu. In order to protect her, Takumi decides to fight again as Kamen Rider Faiz, defeating them, but in turn, Jin reveals himself as Kamen Rider X, attacking and defeating Faiz before claiming that he must find his resolve by himself. The next day, Takumi and Mari are attacked by the Badan Empire. Takumi then confides in Jin about how he is haunted by the death of his friend Masato Kusaka and Jin convinces him to stay at his clinic for a while. Takumi then follows Jin to his clinic, where he helps him tend to another patient. Keisuke Jin appears and heals his wounds before convincing him to release Mari and surrender to the police. Some time later in a ramen shop, Takumi involves himself in an incident where a wounded runaway criminal holds Mari, a high school girl, hostage. Kamen Rider Fifteen appears to attack them, and Tsukasa transforms into Kamen Rider Decade to fight him, allowing Takumi and himself to escape harm. Leaving Kaito behind to help Shotaro with his errand, Tsukasa contacts Takumi Inui who also refuses to help, claiming that his days as a Kamen Rider are over. The first Rider they contact is Shotaro Hidari, who refuses to hear their plea before he finishes his job to search for some lost pets. Tsukasa then departs to gather the other Kamen Riders with Kaito Kumon, while Kouta stays behind to take care of Shu. Before leaving, he warns them that asking for the Shōwa Riders help is useless as they will refuse to cooperate with them.
After Kouta fails to inquire who Tsukasa is, a mini Narutaki appears before them, affirming that the fifteen Heisei Riders must join forces to stop the Badan Empire. Kouta and Mai take Shu to Drupers where they meet Tsukasa Kadoya.
While Gaim was distracted, the Badan forces surround Shu and Mai forcing Kouta to rush back just before Shu uses his powers to allow them escape the oncoming blasts. Just when Gaim begins to falter against Kamen Rider Fifteen, Kamen Riders 1, 2, and V3 appear to assist. He then uses a Lockseed that allows him to transform into the Rider Arms of the 15 Heisei Riders. However, Gaim is attacked by Kamen Rider Fifteen, who also seeks the boy. Badan then launches an attack on the surface, which draws out Armored Riders Baron, Ryugen, and Zangetsu Shin to fight back alongside Kouta. When soldiers from the Underground Badan Empire appear, Takeshi transforms into Kamen Rider 1 to allow Kouta, Mai, and Shu to escape. Kouta and Mai then bring the boy to the surface before being confronted by Takeshi Hongo, demanding them to hand him over. The two find a boy called Shu, who has the power to manipulate things, but does not remember what he is needed for. Upon hearing rumors about it, Kouta Kazuraba investigates the area with Mai Takatsukasa, only to fall into a hole as well and find themselves in an underground, alternate version of the city.
The film opens with Zawame City ravaged by a fight between the Heisei Riders and the Shōwa Riders, before going back in time to the week before, when workers from the Yggdrasill Corporation are dragged by an unknown force underground.
ToQger and Kyoryuger's Kyoryu Red will be joining the fight to protect world peace.
Badan are using monsters from different series and are trying to continue where Shocker left off. Kamen Rider 1, Takeshi Hongo, is trying to defeat Badan and in order to do that he must defeat Gaim. It seems like the fighting is connected to the Underground Empire Badan. The Riders are usually fighting in order to protect their home, but this time around they are fighting each other.