I'm a college student, and I'm invited to my girlfriend's house for the first time to be introduced to her younger sister, Hina. Hina is plain and serious, and shy, and doesn't say much. That night, we were going to have sex, but as we passed each other in the hallway, Hina suddenly pressed her body against mine, whispered in my ear, "I'm more erotic than your sister..." and slid her hand between my legs. She smiled as if toying with me, and started to show me her lewd nature, repeatedly uttering obscene words that you'd never imagine from her appearance...