Three hundred centuries into the future, humanity endured an apocalyptic war, during which 'Garzelia' plotted to destroy 'Sphirna'. The king of Sphirna, 'Anthem', discovered that Garzelia's supreme commander, 'NARITA', was a descendant of humans from Earth in the 2000s—the present day. Twenty years ago, Sphirna dispatched a warrior, Crescent Knights = Yuria, as an assassin, but she ended up carrying NARITA's ancestral seed, throwing the entire plan into chaos. Now, Sphirna sends a new agent—Crescent Knights = Aya. Born as the ordinary office lady 'Aya Natsuki' on Earth, she lives unaware of her true identity—until her mundane life is suddenly thrust into a war spanning the cosmos.