Oshioki katei kyôshi: Nôpan kojin jugyô

お仕置き家庭教師 ノーパン個人授業

Also known as Punishing Home Tutor: Pantyless One-to-One Lessons

Toshinobu Ishii, a high school senior living alone with his father, returns home to find a strange man and woman having sex. Though initially taken aback, Toshinobu finds himself captivated by the sight of them—only to discover that the woman is his own private tutor.