Shuuto Sakaguchi plans to revel in his newfound solitude for the next three months, as his parents depart on a business trip. But when he arrives home from school, he is met by an unfamiliar young girl. This surprise visitor is Chiho Norita, the daughter of one of his parents' colleagues.
To his bewilderment, Shuuto is assigned to look after Chiho without any prior consultation. Unfamiliar with childcare, he must now assume responsibility for her. The two of them spend the next hundred days together, forging a deep connection as Chiho becomes an indispensable part of his everyday life.