The events I'm going to relate happened fifteen years ago. I had recently turned sixteen and it's fair to say I was a bit of a geek. If you wanted someone to help you complete Grand Theft Auto on the PlayStation, I was your man. Experience of girls, not so much. Bit of a cliché I know, but it's who I was: fairly bright, into computer games and role playing but never had a girlfriend. I didn't cut an imposing figure either: a skinny five foot ten with acne scars, I wasn't exactly a prize catch.

Emily was a different matter. The older sister of my best friend Tom, she had long been the unattainable object of my desire. A slender body that curved in all the right places and a cleavage to die for. I lusted after her in the privacy of my bedroom, but asking her out was unthinkable and, even if I had somehow plucked up the courage, I would have been given short shrift. Emily was eighteen and on the rare occasions that she noticed my existence, the look on her face made it clear she saw me as a lesser life form.

On that fateful day I called round to see Tom as I did three or four times a week. I don't remember the reason for my call - probably we were going to play computer games. At first there was no answer when I rang the doorbell but, just as I was about head off, the door opened and Tom's mother looked out.

"Hello Kevin, how are you?"

"Hello Mrs Winterburn, I'm fine thank you. Is Tom home?"

"He's just gone down to the shops, but I think he'll be back in a few minutes. Would you like to come in and wait?" It wasn't as if I had a busy diary filled with people vying for my time. I accepted and Mrs Winterburn ushered me inside.

I followed her into the kitchen, and there I saw a sight that still to this day is seared into my brain, a vision that has brought me much pleasure over the years. Emily, Tom's older sister, was bending over the kitchen table. Her legs were spread about shoulder-width apart, her knickers were pulled down around her knees and her skirt was pushed up over her waist. She was lent right across the table, holding onto the far side.

I stopped right there in my tracks, mouth hanging open like an idiot in search of a village. Let me describe her. Emily was wearing red shoes with high heels, maybe three or four inches. Her ankles were well-defined, leading to shapely calves: smooth, pale skin. Her thighs were firm, not skinny like a model but more toned and curved. On the inside of one thigh Emily had a small bluebird tattoo. As far as I could see, her pussy was completely smooth with tight lips.

"I expect Tom told you how we discipline disobedient children in this house." Mrs Winterburn said to me. "Have you seen a naked girl before, Kevin?"

The answer to both was "no", but there was no time for me to say anything as at that moment Emily realised I was there and shrieked in horror. She jumped upright, hurriedly pulling her knickers up, and furiously swung round to face her mother.

"How dare you let ... him ... see me like that!" The way she said him spoke volumes: there were things she scraped off the bottom of her shoe that she spoke about in a more kindly tone.

"Young lady," Mrs Winterburn snapped back with a voice that allowed no dissent. "while you live in this house, you follow our rules. Who I and your father invite in is no concern of yours. You know what happens if you break a punishment ... or do I have to call your father and fetch the strap?"

Emily looked aghast. I had no idea what was going to happen of course, but she clearly did. She stared at her mother, seemingly unable to believe the words she had just heard.

"Now." Mrs Winterburn commanded. The fight seemed to drain away from her. Eyes dropped to the floor, Emily could only mumble "Yes, Ma'am."

"Would you like a glass of water, Kevin?" It was my turn to stare - after the events of the previous minute such a banal question seemed utterly absurd, but I realised it meant I was being given permission to stay and see what happened next. "Yes please." I replied.

Meanwhile, the gorgeous Emily shuffled around the kitchen, gaze firmly fixed on the tiled floor. Perhaps she was trying to pretend I simply wasn't there - whatever it was I don't think she looked at me once after that. Not just my face - I believe she avoided even seeing my shoes.

First she took a metal bar that had been leaning against a corner. Looking again I saw it was a pull-up bar for keep-fit enthusiasts, but instead of fixing it near the top of the doorway as you'd expect, she fastened it at waist height, a little below the half way point of the doorway through to the pantry.

Then she undressed. Off came her tight midriff-revealing T-shirt. Down came the short thigh-length skirt. Then, right in front of my eyes, she took off her bra and revealed her full, heavy breasts, firm and high - the benefit of youth. I remember thinking how dark her nipples looked against the pale flesh that surrounded them. Finally, she tugged her knickers down and kicked them off. She stood upright, hands by her sides, completely naked apart from her shoes, eyes on the floor. I was utterly transfixed - I don't think anything on Earth could have persuaded me to shift my gaze at that moment.

Mrs Winterburn handed me a glass of water before turning to her daughter. "You may assume the position." Emily turned round and, as I gazed again at her gloriously perfect backside, she stood up to the pull-up bar and bent across it, leaning over to grasp her ankles with both hands. Her breasts swung down and her bottom was as tight as it could be. I drank in the sight, dimly aware that I had a raging erection.

I don't know how long I stood there, the glass of water undrunk in my hand. Sometimes I think it was just a couple of minutes, on other days my memory makes it seem more like ten or fifteen. However long it was, the spell was broken when Tom came home and walked into the kitchen with a bag of shopping.

"Hello Sis." He said in an amused voice, raising an eyebrow in my direction.

"Time you two were off." Mrs Winterburn told us. It wasn't a suggestion and, regretfully, I left the kitchen with Tom.