After his parents' divorce, seven-year-old Koyomi Hidaka chooses to live with his father, a researcher at the Japanese Institute of Virtual Science exploring parallel worlds. Meeting Shiori Satou, the daughter of another researcher, they form a close bond. Despite planning to marry, their parents' unexpected union as step-siblings disrupts their dreams. Determined to defy fate, Koyomi and Shiori flee to a parallel world using their parents' work, igniting a risky journey that jeopardizes their envisioned futures.