On the day of his graduation, Itsuki Yuge learns that his cousin and childhood friend Mahiru is pregnant. But before anything can happen, Itsuki and Mahiru are taken to the magical world of Granvania, which is currently being invaded by monsters. The only people who can fight and exorcise the monsters are the Star Children," mystical beings that can only be produced by the 12 Shrine Maidens of the Constellations. Itsuki has been chosen to father the Star Children with the Maidens. If he wants to get back home with Mahiru, Itsuki must step up to be a man, but how will he be able to manage a host of twelve ladies?