A high-stakes baseball-kun match to become the new brother and sister duo! The sister is Umi, the brother is Riku (a virgin). Recalling the days they spent together since childhood, the two reminisce—Umi says, "We even bathed together..." but Riku jokingly replies, "Nah, I don't get turned on by my big sis lol." However, he's clearly flustered by his unexpectedly mature sister's body. Maybe he takes back his words too quickly—his cock is fully erect. Blinded by a prize large enough to fund five family vacations, the sister lets her brother pop her cherry. Riku thrusts hard and deep inside her, hitting her cervix repeatedly, making his sister orgasm over and over. Even after he cums inside her once, it's not enough—Riku demands a second round. What will become of this brother-sister relationship!? The hilariously wild turn of events will leave you shouting in shock!