I guess I should have known something was wrong. I was as horny as usual at the weekend when her sister and her boyfriend were doing it in the next bedroom to ours, the rhythm of the bed creaking getting faster and faster as the minutes and hours passed away, but I just couldn't get my wife interested. I had my hands down her knickers, my cock pressed between her bum cheeks, my lips down on her clit, but still she didn't get going. Then today, I discovered some soiled lace knickers in her handbag. She had been out last night, to visit her girlfriend from work, or so I thought, and I realised that while I had been getting off watching her sister doing it over the last couple of weeks, she had been getting some real action elsewhere. Now I'm haunted by the thought of where she did it, and who she did it with. Did she enjoy it? What did she wear? I want to see her with her knickers down around her ankles. I'm shocked and yet I'm excited. More excited than ever I was about watching her sister.