A serious and innocent teenage girl named Haru struggles with feelings of isolation as she fails to keep up with her classmates' risqué conversations. Determined to resolve her confusion, she decides to seek guidance about sex and visits her homeroom teacher at his home on a weekend. However, the teacher is a lecherous man who frequents brothels every Sunday, and was just about to head out to one when Haru arrives. Surprised by her persistence, he decides to indulge her curiosity, beginning with explanations about physical development. Trusting his words, Haru gradually allows the teacher to train her young body with his hands. Though pure-hearted deep down, Haru is slowly drawn into a world of forbidden pleasure.