To Every You I've Loved Before

After his parents' divorce, Koyomi Takasaki, who lives with his mother, enters a local high school. He attempts to make friends in his new school, but his social awkwardness combined with an atmosphere of focusing on studies means he is unsuccessful. But one day, he is suddenly approached by his classmate Kazune Takigawa. She says that she has moved from the 85th world and that Koyomi and Kazune are lovers there. Who is Koyomi? Is Koyomi in the parallel world himself in this world?