Itsuki's cousin Mahiru tells him that she's pregnant, on his high school graduation day. Just after they try and figure out what to do a mysterious gate of light emerges and transports the two of them into the world of Granvania. Granvania is a land where "impurities" have been causing major disturbances to its residence, ultimately plunging Granvania into chaos and disorder. Itsuki has been chosen to meet with the "Star Maidens", and is seen as Granvania's last hope. He must produce "Star Children" with the "Star Maidens" to combat the "impurities". If he refuses this destined path, there will be no way for him or his cousin to ever go home.