High schooler Itsuki Yuge is pulled into the parallel world of Granvania alongside his cousin Mahiru, where a floating mascot called Mana and the local court greet them as summoned "Visitors". Granvania is overrun by Impurities that humans cannot fight directly, so Itsuki is tasked with bonding with twelve Star Maidens, each marked by a constellation, to produce Star Children who can battle the threat. As he meets the maidens one by one, returning home depends on clearing Granvania's labyrinths with the children they create.