Waka Onoe and her older brother are seemingly close siblings—but in reality, they're perverted ones. Obscene talk is normal in their household, with daily conversations casually revolving around dicks and pussies. One day, Waka asks her brother, "What exactly gets mixed together in mixed bathing?" He replies, "It's sexual desire mixed with tit lust. That's why it's called mixed bathing, duh." Surprised by his answer, Waka exclaims, "Really? Then I wanna go mixed bathing alone with you, with no one else around!" And so, the two head off to Kyo ni Yuzou, a popular hidden spot known for mixed bathing.