A junior high school sister who still can't stop sucking her fingers. The gesture seemed cute when they were children, but as she grows up, it becomes erotic and confusing to her brother. One day, when the sister sees her brother's erect penis, she jumps in, saying, "I want to suck it." The brother then finds out that his sister has a special constitution that makes her pussy wet just by sucking it, and he can't contain his excitement and they end up having sex. Before long, the sister becomes so addicted to sucking that she gets wet even when he brushes his teeth...