After his parents separate, seven‑year‑old Koyomi Hidaka decides to live with his father. The father works as a researcher at the Japanese Institute of Virtual Science, where he studies phenomena related to parallel worlds. Through his job, Koyomi meets Shiori Satou, the daughter of another researcher who has also gone through a divorce. The two quickly become close friends and spend most of their childhood together.
As they grow older, Koyomi and Shiori plan to marry. Their plans are interrupted when their parents decide to wed, making the children step‑siblings. The pair reject this new reality and use their parents" research to escape into a parallel world, hoping to start anew together. However, the repercussions threaten the very futures they had imagined.
