A high-stakes baseball-knuckle match between siblings. The older sister is Shio Sato, a 23-year-old office lady, and the younger brother is Takumi, a 21-year-old virgin. He jokes, "I don't think I'll get an erection even if I see her naked, haha," and adds, "I won't get turned on—it's my sister, after all, haha." Unaware that such logic won't hold now that both have matured into adults, they dive into the challenge. The result is unexpected: the brother's cock erupts into a massive, rock-hard erection, so shocking he does a double-take. Understandable for a virgin, but clearly, he's fully aroused. Even more surprising, the sister, blushing at her brother's stiff cock, becomes sexually excited in return. From there, the situation escalates uncontrollably into intense incestuous sex. While it seems like a positive development—him losing his virginity and their sibling bond deepening—one can't help but wonder how this will affect their family dynamics going forward. What started as a variety-style prank has now turned into a documentary project following this brother and sister's evolving relationship.