One morning, a middle-aged man runs into Amu, the girl who lives next door, in the elevator—his first time seeing her in a while. That evening, he visits her apartment with a small gift. Their conversation flows casually as usual, but as they chat, he can't help but notice Amu's large breasts peeking through the gap in her shirt. Before he realizes it, his fingers slip inside. Known in the neighborhood as a serious and proper girl, Amu surprisingly doesn't resist—instead, she reacts in an unexpected, almost instinctive way. The moment escalates naturally, and before long, they end up having sex.