Shinji is a little unhappy that his wife and he are going on a trip to a hot spring just for the two of them, but her younger sister Ena joins them. However, when his wife suddenly has to leave the inn for work and they are left alone, she comes to him for advice... This leads to the two of them becoming closer and closer. However, he is unable to resist her devoted advances and they end up having sex... Since they have not had sex in a while, Shinji tells himself that it will only be once. However, Ena doesn't care and sneaks into his bed...