Kimi o Aishita Hitori no Boku e

After his parents' divorce, Koyomi Hidaka, who lives with his father, works at his father's research lab. He meets a girl named Shiori Sato at the laboratory, and they begin a relationship that slowly blossoms into love. But everything changes when their parents decide to marry each other, and they decide to escape to a parallel world where they are not step-siblings.