During a visit to his girlfriend's house to do a report, Nahoto was surprised to see her older sister Onee-chan appear in her pajamas without wearing any underwear, revealing two nipples that looked very arousing, causing him to feel aroused. fascinated from that moment. That night, after being invited to stay for dinner, while going to get water for his girlfriend, he saw her again with an outfit no less provocative than before. This time he couldn't control himself so he secretly took a peek at the exposed part of his chest but unfortunately she discovered it. The lust began to rise, she continuously used sensitive words and even actions such as touching the penis, causing him to not know what to do other than stand still and enjoy, forgetting to get water before. . From now on, he began to learn more about adult matters by having secret love affairs with her behind that poor girlfriend's back.