In a world where people frequently shift between slightly varied parallel universes, Koyomi Hidaka lives with his father post his parents' divorce. At his father's research institute, he encounters Shiori Satou, and their attraction grows. However, their situation takes a drastic turn when their parents decide to remarry. Hopeless that they'll ever be able to pursue their romance, Koyomi and Shiori endeavor to enter a different world where they won't be step-siblings. To Koyomi, a world without her is pointless.