Akagi Kanetake, a boy born into a declining lineage of mages, accidentally activates a magical artifact resembling an hourglass at his grandmother's house one day. When he wakes up, he finds himself in the midst of the Great War, in the Imperial Capital of Tokyo in 1945. Together with a girl he meets there, Fujimiya Tsukumo, Kanetake is dragged into another war being fought behind the scenes of the Great War──the "Holy Grail War"….