Bokuai Kimiai Movie: To Me, The One Who Loved You

Koyomi Hidaka is a boy in elementary school who lives with his father after his parents' divorce. One day, he meets a girl named Shiori Sato at his father's workplace. The two of them begin to fall in love, but then they learn that their parents are going to marry each other, so they decide to run away to a parallel world in which they aren't destined to become brother and sister.