"Rumble Garanndoll" is set in a near-future Japan where a sudden dimensional rift has caused the country to split into two parallel realities: the "Real World" and the "Akatsuki World." The Akatsuki World is a technologically advanced, authoritarian society ruled by the Akatsuki Shogunate, which seeks to conquer the Real World. To combat this threat, the Real World forms a resistance group known as the "Garan Company," which utilizes giant humanoid mechs called Garanndolls to fight back.

The protagonist, Hosomichi Kudou, is a former actor who once starred in a popular tokusatsu series. After the dimensional rift, he finds himself disillusioned and working as a mechanic for the Garan Company. However, his life takes a dramatic turn when he is forced to pilot a Garanndoll named "Arahabaki" to protect the Real World. Hosomichi’s journey is marked by his struggle to reconcile his past as a fictional hero with his newfound role as a real-world defender. Alongside him are other key characters, including Yuki Aoba, a skilled pilot and member of the Garan Company, and Rin Akagi, a mysterious girl with ties to the Akatsuki World.

The narrative explores themes of identity, heroism, and the clash between tradition and modernity. As the Garan Company battles the Akatsuki Shogunate, Hosomichi and his allies uncover deeper secrets about the dimensional rift and the true nature of the Akatsuki World. The story balances intense mecha battles with character-driven moments, delving into the personal struggles and motivations of its cast. The series also incorporates elements of Japanese pop culture, particularly tokusatsu and mecha genres, paying homage to their tropes while crafting a unique narrative. Through its blend of action, drama, and speculative fiction, "Rumble Garanndoll" presents a compelling tale of resistance and self-discovery in a fractured world.