One time when he went to his girlfriend's house to do a report, Nahoto was surprised to see her sister Onee-chan appear in her pajamas without any underwear, revealing her two very stimulating nipples, making him fascinated from that moment. That night, after being invited to stay for dinner, while getting water for his girlfriend, he saw her again in an equally provocative outfit. This time, unable to control himself, he secretly glanced at a part of her exposed chest, but unfortunately, she discovered it. His lust began to rise, she continuously used sensitive words and even actions such as touching his penis, making him unable to do anything but stand still and enjoy, forgetting about getting water earlier. From then on, he began to learn more about adult matters through secret love affairs with her behind his poor girlfriend's back.
