During a visit to his girlfriend's house to do a report, Nahoto was surprised to see her older sister, Onee-chan, appearing in her pajamas without wearing any underwear, revealing her nipples, which looked very arousing, making him was 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 was discovered by her. 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 but stand still and enjoy, forgetting to get water before. . From now on, he began to learn more about adult things by having secret love affairs with her behind that poor girlfriend's back.