Kobeni Yonomori, a typical adolescent, is surprised on her 16th birthday with a fiancé and a sister-in-law she had no idea she had. Hakuya Mitsumine and his younger sister Mashiro have relocated from their farm home to the Yonomori household as a consequence of an agreement made by her late grandfather in order to strengthen their bond with their new family members. Mikakunin de Shinkoukei follows Kobeni's "love life" with Hakuya as she attempts to adjust to the rapid changes that have been thrust upon her. However, when some amazing facts about the siblings emerge, Kobeni's life will be permanently altered.