With his parents going on a business trip, Shuuto Sakaguchi prepares himself to live in complete freedom for the following three months. However, upon returning home from school, he is greeted by a young girl he has never met before. As it turns out, she is Chiho Norita, the daughter of Shuuto's parents' colleague.

Without a say in this matter, Shuuto is suddenly entrusted with taking care of Chiho. However, he has no idea how to take care of children! Having to spend one hundred days together, Shuuto slowly develops a strong bond with Chiho as she becomes an integral part of his daily life.