Rider Great Assembly! Revealing the Mystery of the Forest!

Rider Great Assembly! Revealing the Mystery of the Forest! (ライダー大集結！森の謎を暴け！ Raidā Daishūketsu! Mori no Nazo o Abake!) is the tenth episode of Kamen Rider Gaim. It features the TV debut of Ryugen Kiwi Arms following its first appearance in Kamen Rider × Kamen Rider Gaim & Wizard: The Fateful Sengoku Movie Battle, as well as the debut of Takatora Kureshima's first on-screen transformation into Kamen Rider Zangetsu.

Plot
Christmas is fast approaching and while doing a part time job, Kouta finds two Inves attacking a woman and rescue her before finishing them. With the increasing incidents involving the Inves, Kouta and Mitsuzane decide to investigate the Helheim Forest to look for clues, but they must beware of the white Armored Rider. Mitch then comes up with an idea, by inviting Kaito, Ryoji and Hideyasu for a game in the forest, with the team catching the most Lockseeds declared as the winner. As Team Gaim is the only team with two Riders, Kouta agrees to sit out of the game, as his true intention is to have the other Riders distracting the white Rider and his minions while he looks for clues about the Inves and Yuya's whereabouts. To give Ryoji and Hideyasu access to the forest, the Riders convince Sid to provide them some Lock Vehicles but he claims that he will have them ready by Christmas. Mai gets angry at Kouta and Mitch for proposing such game just when Team Gaim is on top of the rankings but they calm her down, claiming that it is necessary for them to learn the truth about the Inves and they promise to will return in time for their Christmas party. On the day of the game, the Riders are about to start when Oren appears, convincing them to join after he reveals that he also possesses a Lock Vehicle. However, once several Inves appear before them, Kamen Rider Baron runs ahead of the others to the forest to have an advantage, with Bravo following soon after. To delay Ryugen further, Kurokage and Gridon feed a Himawari Lockseed to an Inves to strengthen it and rush to the forest as well. With all other Riders away, Gaim appears to assist Ryugen who fights the Inves as the new Armored Rider Ryugen Kiwi Arms and instructs him to run after the others. Before leaving, Ryugen lends Gaim the Suika Lockseed to help him dispose of the enemies. Upon knowing of the Riders appearing at the Helheim Forest, Takatora transforms into Armored Rider Zangetsu as he is about to depart there as well.