I hadn't seen Lisa since our university days, six years ago.

She arrived, as only Lisa can, a cloud of perfume, outrageous heels and the latest designer gear. My husband, Bob, did what most men do when they first meet Lisa, stared, open-mouthed, totally gobsmacked. She is, after all, stunningly beautiful.

That night we did what Bob and I hadn't done for years and went on a tour of the town centre pubs. Even Bob seemed to enjoy himself, which is a revelation, and Lisa is still fantastic company and a great laugh.

Back home, and Bob did what he normally does and fell asleep in his chair. And Lisa and I turned back time to our university days, poured two glasses of wine, switched off the light, sat on the sofa and talked.

For all her flamboyance Lisa has always been a good friend and a good listener and, probably because of the alcohol mixed with a lot of self-pity, I found myself opening my heart to her. My marriage to Bob was a sham and had been going steadily downhill for years. There was no one else involved, I'd checked up on that, but Bob was obsessed with work. He's a solicitor and there just doesn't seem enough hours in the day he even brings work home. We have no social life and sex is non-existent.

Lisa listened patiently and when I started to cry, quietly, she took me in her arms and stroked my hair. Once, when we were at university, Lisa had made a, booze induced, pass at me, but I'd pushed her away and laughed it off. It's not my scene, but Lisa, being Lisa, swings. So when she gently kissed the top of my head I just went with the flow. Lisa must have sensed my willingness, more contentment really, and continued. It was all very bizarre, for me anyway, but tremendously exciting, even more so with Bob, gently snoring, in the same room. So when she slipped down and got her head between my legs I just let her. Well actually, it was more than just letting her, I wanted her, by this time, very badly. The orgasm, when it came, was mind blowing, she'd certainly done it before and I had to bite down on my bottom lip to stop me crying out. A cry of utter frustration and electrifying excitement.

And then I did something I can honestly say I'd never thought, in my wildest fantasies, that I'd ever do. I returned the favour. Now, it was my turn, on my knees. She smelt of musk, and perfume, and wetness and I was lost in the total excitement of it all. Wrapped up in total abandonment. That's when I heard Bob move. Lisa, moaning softly, must have heard him too, because we both froze, terrified. We held our breath as Bob seemed to resettle himself in the chair. And then, only because Lisa pushed my head into her crotch and I sensed her desperation, did I continue. Now, terrified that he would wake up, I just wanted Lisa to come, quickly.

Then, suddenly, there was a sound of springs and movement and Bob was there, on his knees, behind me. I lifted my head from Lisa's crotch, petrified. And then a very weird thing happened. Bob pushed my head down gently and said, very excitedly, "Don't stop." And I didn't. And it was all erotic, and weird and, yes, wonderful. Bob was inside me with a desperation I never thought he possessed. Pounding into me like someone demented and I too, caught up in this erotic fantasy, wanted him like I'd never wanted anyone in my life. Like a high voltage charge it couldn't last and Bob shuddered again, and again into me. And I too thrust back into him crying out in unison as I came over and over. And then Lisa reared up arching her back and thrusting her crotch into my face. We collapsed in a pornographic heap.

The next morning Bob and I both felt a bit embarrassed, but Lisa just behaved as if nothing had happened. Maybe for her it was just par for the course? Then a rather wonderful thing happened, with Lisa gone Bob made love to me, it wasn't having sex, it was gentle, loving, love. And it was wonderful. And, amazingly, it's continued. Bob still works very hard, but we have a life now. We go out for meals, see friends we haven't seen for years and the sex is great. Last week, you wouldn't believe it, he actually bought me flowers.

Next month Lisa is coming up again for the week-end and, although we haven't talked about it, I know Bob's very excited. I know too that we are bound to have another threesome and I realize that Bob's going to fuck Lisa this time, it's inevitable. And, weirdly, one part of me is very excited too. But the other part, I know, will be insanely jealous.

Can a marriage survive such a threesome? Or our we all on a slippery slope to nowhere, pain and heartache?