In Zawame City, the Yggdrasill Corporation are transforming the feeling of a once bustling suburban city into a jokamachi (Castle town). To escape this feeling, young people throughout the city have formed dance crews called Beat Riders to bring joy back into their lives. The Inves Game, a simulator that uses Lock Seeds to summon Inves monsters, is rising in popularity. Kouta Kazuraba, a member of the dance group Team Gaim, is trying to find his place in the world while torn between his loyalty to his teammates and his sister Akira's insistence that he find a job. When Team Gaim's captain disappears after informing Kouta of a strange belt he found, Kouta discovers that the Lock Seeds and Inves are from an alternate dimension known as Helheim Forest. When he is attacked by an Inves, Kouta uses the Sengoku Driver belt and the Orange Lock Seed to transform into Kamen Rider Gaim to destroy the Inves while encountering a mysterious girl resembling his friend Mai Takatsukasa who tells him that his fate is sealed. He soon discovers that he can also battle within the Inves Games, and makes a name for himself.

In time, other Armored Riders appear, all vying for social power throughout town, including Team Baron's leader Kaito Kumon as Armored Rider Baron, Kouta's Team Gaim teammate Mitsuzane Kureshima as Ryugen, Team Raid Wild's Ryoji Hase as Kurokage, the scheming Hideyasu Jonouchi of Team Invitto as Gridon, and the perfectionist ex-soldier Oren Pierre Alfonzo as Bravo. Unbeknownst to them is that Mitsuzane's older brother Takatora Kureshima, the head of R&D at the Yggdrasill Corporation, is the enigmatic white Armored Rider Zangetsu who is protecting a dark secret hidden in Helheim Forest at any cost while using them and Inves Games for research. Ultimately, when Kouta confronts Takatora as he and members of his company become New Generation Armored Riders Duke, Sigurd, and Marika, he learns the horrible truth of Helheim Forest: it was home to a once bustling civilization that was eradicated by the strange plants growing throughout it, as anyone who consumed the fruit of these plants was turned into an Inves. Finding an ally in DJ Sagara, a pirate radio DJ who covered the Beat Riders and is privy to knowledge about both Yggdrasill and Helheim, Kouta finds himself in the middle of a Sengoku period-like brawl among various factions for the right to possess the power of Helheim Forest and the fate of life on Earth as he knows it.