On the day of his high school graduation, Itsuki Yuuge learns a shocking truth – his cousin and childhood friend Mahiru is pregnant. Their world suddenly shifts as they are whisked away to Granvania, a fantastical realm under siege by monsters. The key to combating these creatures lies with the 'star children,' unique beings born from the 12 'shrine maidens of the constellations.' To protect their homeland and find a way back, Itsuki must join forces with these maidens to sire the star children.