A Certain Undefined Gaia

Gaia City stands as the pinnacle of human advancement—a sprawling, walled metropolis where science has given birth to the extraordinary. Within its boundaries live Gaias, individuals born with Arthem circuits, synthetic pathways infused with Psi Waves and the mysterious energy known as Metra. Ranked from Eta to Alpha, Gaias are classified, studied, and controlled under a rigid system designed to maintain order and suppress the unknown. But science can only explain so much. Beyond the city's borders dwell the Mergus, wielders of pure Metra through forbidden Ancient Scripts and sacred texts long dismissed as myth by Gaia City's researchers. Unlike Gaias, Mergus tap into forces unquantifiable, echoing lost wisdom and primal power. Contact between the two worlds is outlawed—for good reason. Exposure to raw Metra could cripple or kill a Gaia, disrupting their Arthem at the core. Ryouma Winslow, an Eta-class Gaia dismissed as a nobody, had no intention of getting involved with either side. But everything changes when he crosses paths with Archive (Shiro), a strange girl with fractured memories and a presence that doesn’t belong in Gaia City. Helping her seemed simple—until restoring her mind reveals a truth that shatters the world he thought he knew. Now entangled with rogue Mergus factions, hunted by unseen forces, and navigating fragile alliances with powerful Level Alpha Gaias like the electrifying Mei Tokisaki, Ryouma finds himself drawn into a multilayered conflict that transcends science, challenges belief, and threatens the foundations of Gaia City itself. Because in a world split between science and sorcery, the greatest danger is a boy who doesn’t fit into either side. Author's Note to Readers: This story draws clear inspiration from the Toaru series (A Certain Magical Index / Scientific Railgun), and longtime fans will notice structural and tonal similarities in its arcs, setting, and faction dynamics. While some adaptations and elements may feel familiar, please know that the world, characters, power systems, and underlying themes in A Certain Undefined Gaia are entirely original and unique to this work. Think of it in the same way that fans appreciate both Mushoku Tensei and TBATE—similarities exist, but each story carries its own voice, meaning, and direction.