Sarah and I had known Dave for around 5 years - we'd often gone for nights out together and often shared the hot tub late at night. However, nothing saucy had ever happened, except for a few drunken snogs between Sarah and Dave - all in good fun after a few beers. Sarah and I had never been with anyone else and had married when Sarah was 19 and me 24 - we're now 35 and 40. We'd decided against having kids and Sarah managed to keep her weight around 9st and a (UK) size 8. She has small 34a tits and great legs, a tight arse and flat stomach. I, on the other hand had shot up to 18st; though I am 6'4" lol. Anyway, our sex life has been suffering for a while and Sarah had stopped wearing the sexy clothes she once wore - short skirts, high heels, boots etc. Knowing I could trust Dave not to spill the beans, I told Sarah that Dave had asked me if I would be offended if he took Sarah to a works party he was due to attend. I "let slip" that he asked if she had a sexy dress to wear. Sarah was a little taken aback by this "invite" but jumped at the chance to go once I told her I didn't mind it - but that she would need a sexy outfit if she was going. Of course, Dave knew nothing of this until I told him he could borrow her for the night and that she was buying a sexy dress for the occassion. He was over the moon that he would have someone to take with him as he'd been single since divorcing 5 years earlier. Inside, I knew that he wouldn't resist trying to get into her knickers too. I couldn't believe it when she suggested that she would pop up to Bristol that weekend so she could have a look around. On the Saturday night, she put her new dress on and asked me what I thought. TBH, whilst she looked nice it certainly couldn't be classed as a sexy dress at all, but I couldn't tell her that and said she looked great. My mind was working overtime on what to do. In the end I spoke with Dave and told him the problem. Leave it with me he said and told me he would get a dress for her and see if she would wear it; but that he would show it to me first to make sure I was OK with it and it wasn't too daring. Around a week later, and just a few days before his works party, I got a phone call from Dave. He explained that he had been to our house that day, knowing that I was at work and Sarah was at home. She invited him in and asked what he was doing there and not at work. He explained that he wanted to ask her a huge favour and that she shouldn't be angry with him. He told her that he had seen an amazing dress which he though she would look stunning in. He had already purchased the dress and wanted to know what she thought of it. Sarah told him that she had already got a dress but, after a few more compliments finally agreed to try the dress on. Dave handed it to her and she went to get changed. When sarah returned, Daves jaw fell open and he said how amazing she looked. The dress was made of a black satin material and stopped about mid thigh. It practically covered her front but was backless - so much so that the material didn't start until just above her arse cheeks. He could sense that Sarah was unsure of the dress and she told him that she couldnt wear a bra with it. Dave again complimented her on how she looked and suggested that she gave it some thought. He told her just how much he would love to have her on his arm looking so fabulous in the dress, but that he would return it if she decided she couldn't wear it. The Saturday of the party came and Sarah spent ages getting ready. Dave was due to pick her ip at 7.30 and she appeared with 5 minutes to spare. I couldn't believe how hot she looked and my cock sprung to attention and ached to be released. I commented that the dress was different to the one she had previously got and she replied that she wanted something different and had exchanged it - hoping that I didn't think it too sexy. As she walked toward me, it was obvious that she was bra-less as her little titties jiggled around under the fabric. What was also obvious when she turned around, was just how low the back was; to the point where her panties were showing. I explained this to her and she started to panic. "Just take them off" I told her. Before she could say anything, the doorbell rang and I let Dave in. Sarah had left the room but quickly returned. They greeted each other and Dave told her how great she looked and then they moved toward the door. I noticed that she had removed her panties and, rather belatedly, that she must be wearing hold-ups as the band of her tights couldn't be seen. I heard the clicks of her high heels get quieter and then closed the door; knowing that I had just loaned my wife out for the night dressed the hottest she's been for years. Part 2 to follow.