When high school student Hinata Sunohara was summoned to another world to defeat the Demon Lord, he promised to marry Mariabelle, a beautiful dark elf. But once his mission ended, Hinata was returned to Earth, leaving Mariabelle devastated.

After a month back in Japan, Hinata's life has seemingly gone back to normal. However, everything changes again when Mariabelle suddenly appears in his room! Swearing to remain by his side forever, she begins living with him. As her obsessive love grows by the day, Mariabelle wishes to protect Hinata from any threat and ensure that he only has eyes for her.