"Horizon in the Middle of Nowhere" is set in a distant future where humanity has abandoned Earth due to environmental collapse, only to return centuries later to find it drastically altered. The world is now a patchwork of floating continents, each governed by different nations and factions. The story unfolds in the Far East, where the Testament Union, a governing body, enforces a strict adherence to historical reenactments based on the Testament, a sacred text dictating the course of events. These reenactments are believed to restore the world to its former state.

The narrative centers on Tori Aoi, a carefree and seemingly ordinary student at Musashi Ariadust Academy, and his childhood friend, Horizon Ariadust, who is revealed to be the key to unlocking the secrets of the Testament. Horizon, a mysterious girl with a mechanical body, possesses the ability to manipulate the Testament's directives, making her a target for various factions vying for control. When Horizon is kidnapped by the Testament Union, Tori and his classmates embark on a perilous journey to rescue her, navigating political intrigue, battles, and the complexities of the Testament's rules.

The series features a large ensemble cast, including members of the Musashi Ariadust Academy's student council, such as the stoic and strategic Naomasa, the energetic and combative Tenzou, and the resourceful and intelligent Suzu. Each character brings unique skills and perspectives to the group, contributing to their efforts to challenge the Testament Union and uncover the truth behind Horizon's significance.

As the story progresses, the group encounters various nations and factions, each with their own agendas and interpretations of the Testament. The narrative explores themes of loyalty, sacrifice, and the consequences of blindly following tradition. The interplay between political maneuvering and personal relationships adds depth to the plot, as alliances are forged and tested in the face of escalating conflicts.

The series is characterized by its intricate world-building, blending elements of science fiction, fantasy, and historical references. The reenactments of historical events, combined with advanced technology and supernatural abilities, create a unique setting that drives the narrative forward. The stakes are heightened as the characters uncover the true nature of the Testament and its impact on their world, leading to a climactic confrontation that challenges their beliefs and resolves long-standing mysteries.