A young beggar boy named Grimm lives in a conquered territory of the mighty Geina Empire. When he stumbles upon a rusted humanoid weapon called a Gear in an abandoned house, memories of his past life come flooding back. He realizes he has been reborn into the world of Gear x Magic, a popular robot anime about humanity's desperate war against half-human, half-machine monsters known as Gargoyles.

Grimm knows how the original story unfolds. The protagonist goes berserk, murders the imperial princess, and triggers a chain of events leading directly to the empire's collapse and humanity's downfall. This time, Grimm refuses to let that bad ending happen. Though he was a minor character who never appeared in the anime, he possesses a rare talent as an engineer. He decides to develop cutting-edge Gear technology powerful enough to defeat the original protagonist and save the princess's life through the power of innovation rather than following the predetermined plot.

The world is defined by a brutal struggle for survival. Gargoyles threaten to overwhelm humanity, driving the development of magical power armor known as Gears that have transformed the nature of warfare. The Geina Empire stands as humanity's greatest hope, but internal corruption and the impending royal succession crisis threaten to destroy everything from within. Grimm starts from the lowest possible position, a discriminated orphan in a defeated nation, yet he rises by demonstrating genius-level mechanical skills that earn him the trust of garrison soldiers and eventually powerful aristocrats and military officers.

The central narrative arc follows Grimm's methodical ascent from beggar to imperial engineer. He leverages his encyclopedic knowledge of the original anime to anticipate future events and create technological innovations that leapfrog anything the current era has achieved. His primary goal is to overhaul the second princess Rouge's personal Gear, dramatically upgrading its performance to survive the coming confrontation with the rampaging hero. The story accelerates when Grimm begins encountering characters from the anime, accelerating his timeline as he races against the approaching catastrophe. Despite his growing influence, achieving his ultimate objective remains an immense challenge that pushes his engineering skills and knowledge of the original plot to their absolute limits.