The narrator splurges on a hot-spring trip hoping for sex with his girlfriend, but she brings her adult sister Mio to prevent intimacy. The plan backfires because pure-looking Mio is secretly insatiable. The moment they reach the inn she begins tempting him: “Using my hand doesn't count as cheating… probably.” Despite his guilt, he surrenders to her cute face, whispered dirty talk, and skilled hands throughout the three-night stay, ejaculating thirteen times and falling completely under her control.