In a far-off age, humanity faces aggression from the Abh Empire, a species of highly evolved human-like beings with far superior technology. As numerous planets succumb to the Abh, mankind forms the Four Nations Alliance, a coalition comprising the United Mankind, the Republic of Greater Alcont, the Federation of Hania, and the People's Sovereign of Union Planets.
Seikai no Monshou follows Jinto Linn. As a child, his father—the president of Martine—traded their planet for a prestigious role within the empire. Now a youthful count, Jinto must adapt to the customs of Abh nobility and reside among those who conquered his homeworld. Assisting him is Lafiel Abriel, a stern Abh princess who becomes his fast friend. En route to Jinto's new academy in the Abh homeland, their vessel becomes trapped in a fierce space clash between Alliance and Abh fleets. Jinto and Abriel are thrown into the turmoil, unaware that this battle signals the onset of a full-scale war between the Abh Empire and humanity.