A psychiatrist, Fujishiro Shun, receives a letter from his former lover, Iyota Haru. The letter, which came from Salar de Uyuni, known for the "mirror of the sky," contains the memory of her first love from ten years ago. Uyuni, Prague, and Iceland More letters would soon arrive from all over the world, the letters of spring. At the same time, Fujishiro was preparing to marry his fiancé, Sakamoto Yayoi. But Yayoi suddenly disappeared. "What could be the way to keep love from ending?" Why did Haru write a letter to him? Where did Yayoi disappear? These two mysterious entities would soon become connected. "Why do love and romance, which we thought would last forever, eventually fade away?" The present and the past, Japan and the world, intersect. As "April" begins, he will search for the truth about his loved ones. Adapted from the novel "Shigatsu ni Nareba Kanojo wa" (四月になれば彼女は) by Kawamura Genki (川村元気).