On the day of his high school graduation, Itsuki's cousin Mahiru informs him that she is pregnant. At that moment, a gate of light appears and transports both of them to the world of Granvania. In this land, "Impurities" are disrupting the Stars, which has ultimately thrown Granvania into chaos and disorder. Itsuki is revealed to be one destined to meet the "Star Maidens," and he is viewed as Granvania's last hope. Consequently, he is tasked with producing "Star Children" and fighting the "impurities." If he fails to complete this task, Itsuki may never be able to return home.