Itsuki is hit with a bombshell on the day of his high school graduation when his cousin, Mahiru, tells him she's pregnant. But their shock is short-lived when they are suddenly transported to the chaotic world of Granvania through a mysterious light gate. The land is plagued by Impurities that disrupt the Stars and threaten its very existence. Itsuki discovers that he has a significant role to play - to meet and form a bond with the Star Maidens and produce Star Children that will fight the Impurities. Unfortunately, failure to fulfill his duty would mean that he may never be able to return home.