At the end of World War II, a demon-summoning ritual drastically altered the face of the Earth. One hundred years later, in the year 2055, demons rule the world, and humanity fights back using armored mechs powered by a resource known as magicite. The protagonist, Keita Kawakami, enrolls in Japan's military academy, carrying a unique advantage: he possesses vivid memories of an alternate, peaceful version of the modern era. This knowledge from his previous life allows him to achieve what no one else could successfully synchronizing with and piloting a prototype mech that had been impossible to activate. Thrown immediately from the classroom to the battlefield, Keita uses his unconventional wisdom to achieve results that defy all established norms, turning the tide of battle in ways no one expected.

The setting is a war-torn Japan divided into nine defense districts, each managed by a division of the National Defense Force facing different strategic pressures. The story follows Keita Kawakami, a seemingly ordinary recruit whose past-life memories give him an extraordinary edge. The narrative also includes a cast of rival cadets and officers, such as Kazunari and Junji from the 7th Division, and Shouko Isotani from the 6th Division. These characters are not just classmates but pawns in a larger power struggle between military factions, a conflict intensified by the recent annihilation of the 3rd Division. For them, the academy rankings are a matter of life and death, directly influencing which faction will control the rebuilding of the lost division.

The first major narrative arc, known as The Kyushu Grand Offensive, focuses on the battle against demon tribes invading from the Asian continent. While the official objective is to reclaim territory, a subtext of political maneuvering runs through the campaign. As Keita deploys to the Kyushu-Chugoku region, his actions on the battlefield attract the wary attention of these rival factions, who see his anomalous success as a potential threat to their influence. His journey thus becomes a battle on two fronts: a physical war to reclaim the world from demonic rule, and a political war where his very existence challenges the established norms of a military hierarchy struggling to survive.