Mama wa Dōkyūsei

Kisaragi Ryou lost his mother at a young age and with his father away from home most of the time, he is left living at home alone often. One morning, he discovers a naked women in his house. Without being able to assess the situation, he heads off to school. Later that evening, his father comes home to introduce him to the girl he plans to remarry and this girl turns out to be his classmate.