So, I guess it all began as we sat under a tree by the lake during the last few weeks of our sixth year at Hogwarts. The bright summer sun was blazing down, the heat tempered slightly by a warm breeze, causing the susurration of the leaves that dappled the ground with light and shade.

Ginny and Harry had finally revealed to Ron that they were a couple and after a few days of sulking and grumbling, he had, at last, begun coming to terms with the fact that it was going to happen whether he liked it or not. Ron and I were closer than we had ever been. His entanglement with Lavender was completely over and I think he was finally beginning to realise that what we shared was something deeper than just friendship.

As we lay around enjoying the warm breeze that drifted off the lake I can remember the feeling of contentment mixed with a knot of dread that these sunny days were coming to an end. Although we hadn't discussed it I was pretty sure that Harry was thinking of leaving Hogwarts at the end of the year, and as was always the case, he would feel he needed to do it alone, idiot boy that he was. Even Professor Dumbledore couldn't change Harry's mind once it was made up.

Of course, we needed to do something. Voldemort was gaining power rapidly. We knew that the Ministry of Magic was pretty much in his hands now and his Death Eaters were creating havoc once more. But I was sure Harry charging off on his own to look for Horcruxes wasn't the answer.

For some time, I had been considering the events that led to where we were today, right back to Voldemort's first rise to power and trying to unravel in my mind how people's actions had affected things. Eventually, it had become obvious, to me at least, that the person central to everything was our Potions Master, Professor Severus Snape. In truth, Professor Snape was at that time our Defence Against the Dark Arts teacher, a post he had apparently coveted since joining the school but I still couldn't help thinking of him as the Potions Master as he had been that for so long.

Harry had told me once, in a fit of panic, something he had learnt about Professor Snape during the Occlumency Lessons he had taken with the snarky Potions Master back in the fifth year. Given an unexpected opportunity, Harry had broken Professor Snape's trust irrevocably by looking at a memory the teacher had placed inside a Pensieve before the lesson began. The memory contained images of Harry's parents and didn't show his father, in particular, in a good light. Worried that Professor Snape had been right about his father being a bully, Harry had told me all about the memory and Professor Snape's reaction and I had promised on pain of death never to repeat it to anyone else.

The seriousness of the memory was shown by the fact that Harry didn't even tell Ron about it, which I have to say relieved me greatly; with his hatred of Professor Snape I'm sure Ron would have ended up blurting it out when he lost his temper as he's never been the most tactful of people.

But the memory, and in particular Professor Snape's reaction to Harry seeing it, had got me thinking.

It was clear from what Harry had described that his mother and Professor Snape had at one time been, at the very least, close friends. Harry was blinkered by his hatred for Professor Snape and didn't want to believe that to be the case, but I could easily imagine it. And the way Harry had described the hurt that Lily had shown when Professor Snape called her a Mudblood — well, it would have been the same as if Ron had said those words to me.

The more I considered it, the more convinced I became that there had definitely been something between Professor Snape and Lily although by the time they left school the relationship was over and she was with James Potter, who she had gone on to marry. From there it wasn't too great a step to wondering whether maybe Professor Snape had been in love with Lily and the reason the memory had been so painful for him was because it showed the end of his relationship with her. Had he carried a torch for her after they left school?

As I looked at events and pieced things together I became more convinced. Professor Snape had still been in love with Harry's mother, even when she and James died, and somehow this had made him protect Harry even though Harry looked so much like his detested father, which actually, also explained why Professor Snape seemed to hate him so much.

I still wasn't entirely sure how Professor Snape fitted into the death of Harry's parents; a nagging part of me said that he was definitely involved somehow but I was sure it was all tied into the reason that Professor Dumbledore trusted Professor Snape so implicitly and refused to listen to any arguments about the Potions Master still being a Death Eater.

Once I had convinced myself that Professor Snape was the key player in all of this I set about seeing how he tied in with the other main players. That he had been a Death Eater was a well-known fact. As I said, several members of the Order of the Phoenix were convinced he still was, although being fair to the Professor that had been mainly Harry's godfather, Sirius Black and he had been James' best friend and detested Professor Snape at least as much as James had.

But Professor Dumbledore trusted him and whilst it was troubling that some of Professor Snape's actions appeared questionable, to say the least, we had no choice but to do the same. Harry and Ron did so belligerently, but I remembered how in the first year we had misjudged the teacher completely, blaming him for trying to kill Harry when he had in fact, been attempting to keep him alive.

It wasn't easy trying to remember that the sullen teacher was on our side, especially when he was so mean to us in Potions and Defence Against the Dark Arts, and Harry and Ron found it all too easy to blame him for every bad thing that happened. But being mentally somewhat older than my friends I managed to see past that and I was sure that Professor Dumbledore was right. Somewhere deep inside, Professor Snape was good.

From the brief snatches of meetings we had managed to overhear the Order of the Phoenix having at Grimmauld Place I got the impression that Professor Snape was back at Voldemort's side working as a spy for the Order, which pretty much meant that he'd had that role the previous time the Dark Lord, as he was known to his followers, had risen to power although whether Professor Snape had been on our side then was unclear.

Although Harry didn't like to admit it, Professor Snape had spent our entire time at school keeping watch over him and making sure he didn't come to any harm — and there had been plenty of occasions when he might have because Harry just naturally seems to attract trouble.

I still hadn't worked out whether the Professor was working on Dumbledore's orders or was doing it because of his love for Lily. I was fairly certain he knew of the prophecy that had been made about Harry being the Chosen One. That seemed to be fairly common knowledge these days. But more important was when he had first learnt of the actual contents. Had he been aware of it during Voldemort's first rise and realised that the prophecy referred to Lily's son? If he had, maybe it would explain his behaviour towards Harry. Hating him for looking so much like his father, Professor Snape would still feel compelled to safeguard him against the future rise of Voldemort to make up for . . . what . . . letting Lily down and allowing Voldemort to kill her?

I knew that was probably a little too fanciful, even for me, but the prophecy definitely fitted somewhere in that hazy mess between the Potters and Professor Snape.

Back in my third year at school, Professor Dumbledore had agreed that I should be given a Time-Turner so I could study more lessons than everyone else. In retrospect, it was a terrible idea. Even with my organisational skills I couldn't keep up with the extra work and argued with both Ron and Harry over the course of the year, nearly losing their friendship completely along the way.

In fact, it was only Hagrid who kept me sane during those tense, unhappy days, which is why I hit Draco Malfoy when he took the piss out of Hagrid for being upset at the destruction of his Hippogriff, Buckbeak; though if I'm honest I had been tempted to do it several times before that as the obnoxious prat had been winding me up ever since we first started at Hogwarts.

Although the Time-Turner eventually proved useful in allowing Harry and me to free both Buckbeak and Sirius, at the end of the year I decided to give it back and return to a more normal level of study and regain my sanity.

In our fifth year, we fought Voldemort's Death Eaters at the Ministry of Magic during which all the Time-Turners were destroyed, or so it was reported in the Daily Prophet. But Harry, who was forever in the Headmaster's study for one reason or another, had told me a few weeks earlier that he had seen my Time-Turner on a shelf in a cupboard with the Sorting Hat. For some reason, the Headmaster hadn't returned it to the Ministry of Magic.

That got me considering how we could use the Time-Turner to go back to before Harry's parents' deaths — back to when they and Professor Snape were still at school but sometime after the time of the Potions Master's memory, to see what had happened to make Harry into the Chosen One — and then change the past so his parents didn't die. Without the prophecy coming true, time would take a different path . . . a better path.

Obviously, I would have to be the one to go. Harry couldn't do it as it would be his parents he was dealing with and that would make things just too difficult, even if you forgot the fact that he looked exactly like a younger version of his father and had his mother's eye and I knew he would have trouble seeing Sirius again, even if he was twenty-odd years younger. Ron, too, with his bright red Weasley hair, was out of the question. Although his parents had left the school by that time he was far too noticeable to pass without questioning. And as much as I loved Ron, I didn't think his skills at bluffing were that good.

I, on the other hand, was perfect for the task. An average-looking and, when my hair was tied back, rather nondescript Muggle-born teenage girl with higher than average intelligence and the ability to cast spells far beyond my years, I could infiltrate the school without really being noticed. And if I ended up being there for longer than anticipated and had to sit the N.E.W.T. exams I was certain I could do so fairly confidently, not something that either of my two friends could say.

Therefore, I spent those few weeks drawing up intricate plans of what I thought had happened all those years before, what I thought I could do to put the situation right and finally, timetables of what Harry and Ron would need to do in order to help me successfully implement the plan. Because, as with everything we had achieved over the years, we had acted as a team and this time would be no different.

So, as we sat on that balmy summer's day, I ran through my head one last time exactly what I was going to say to get them to help me with my plan. I knew that once I managed to get them properly listening to what I had discovered and forced the information through their thick skulls and into their brains, the boys would agree to help me, if only because there wasn't really anything else to try . . . except for finding Horcruxes.

I did briefly consider taking my plan to Professor Dumbledore and asking his opinion. But it seemed to me that over the years he had left Harry in the dark about too many important discoveries to be completely trusted. Also, I knew that if he said no to my plan, the frustration of knowing I could do something to change history and not being allowed to do anything about it would be overwhelming.

And so the plan had to remain a closely guarded secret.

There was a pattern to our days in the sun. Ron, always keen to forget about schoolwork, would lie on his back with his hands behind his head talking about the summer holidays as if we had nothing more troubling in our lives. Sometimes I envied him, that he could so effectively ignore all the important and bad stuff although most of the time it just annoyed me.

Harry would be in agreement with Ron, although sometimes when I looked into those emerald green eyes of his I could see him thinking and worrying, his brain running through a series of complex plans that he wouldn't, or couldn't, share. Just occasionally his mood would turn melancholic, which with Harry meant angry and it could be hard to shake him out of it.

Ginny was just happy to be with Harry. In love with him since the first time she met him, it had been a long haul before Harry finally realised that beautiful Ginny, with her long fiery red hair, wasn't just Ron's little sister any longer. It was hard for her sometimes as, being a year younger than us, she was rarely included in the things we did during school time but on days like the one I am describing she could happily spend her time with us.

Then there was me. I was always thinking about something: school work and exams, the Order of the Phoenix and, as I've just said, the cause and effect of Voldemort. People call me bossy but really I'm just highly organised and like others to be that way, too. That's why I always drew up Harry and Ron's exam timetables otherwise, they would never get anything done.

I was also in love with Ron, which is why, although he drove me completely potty with his laziness sometimes, I always helped him to complete his homework and was so glad that Lavender Brown had finally been given the boot.

As I said earlier, although I was enjoying the sunshine and the chance to do nothing I couldn't relax, not completely. I tried, I really did, knowing that Ron would moan at me for ruining their afternoon if I mentioned Professor Snape or Voldemort. But I was sure time was running out, both for the wizarding world and for Harry, so eventually, I couldn't help but bring the subject up.

I started, as I had tried to do a few times in the past, by trying to get Harry to talk about what he was going to do once school was finished for the year. As ever, my secretive friend was shifty, brittle and noncommittal, refusing to keep eye contact with any of us for more than a couple of seconds. Then Ron piped up and of course, sided with Harry; telling me to stop pestering him.

But I didn't want to drop it. I was eager to get my plan into action and I needed them for it to work.

Eventually, with a hugely impatient sigh, Ron lay back down on the grass shaking his head in surrender. Harry looked at me intently, knowing I wouldn't be so insistent on talking if I didn't think it was important and he, at least, took this as seriously as me. Ginny, who knew trouble was coming, glanced at her brother then rose and gave Harry a brief kiss on the lips before making her excuses and leaving so the three of us could talk on our own.

I started to explain, firstly about my suspicions and discoveries, then I followed up with my conclusions and plans. As I had expected, Harry didn't want to hear about his mother and Professor Snape and Ron had blasted a laugh of complete disbelief at my suggestion. But slowly, as I unravelled my story, Harry became calmer and more interested and Ron even sat up and was looking at my carefully drawn plans. We sat there for another hour, me talking until I was almost hoarse as I answered all the questions that Harry and Ron threw at me, both about my conclusions and the plan and as the sun began to set over the lake I realised I had convinced them.

Central to my plan was the Time-Turner and the acquisition of it had to be left to Harry. He would have to pick a moment when he was left alone in the Headmaster's study and liberate it from its cupboard. I was hoping that Professor Dumbledore hadn't put a charm on the item or we would fall at the first hurdle, but something inside me was telling me this was how we had to proceed so I pushed that thought aside.

Pleased with what I had managed to achieve, I rolled up the parchment. The three of us stood and headed back across the lawn towards the castle, its shadows lengthening as the daylight disappeared into the blue of evening.

Amazingly, things fell into place only two days later when Harry was summoned to one of his special lessons with the Headmaster. When he returned to the common room several hours later, the three of us left to find somewhere in the castle where we could discuss privately what had taken place.

Harry talked rapidly about his time with Professor Dumbledore and his renewed promise to allow Harry to go with him when he located another Horcrux. This was interesting enough news but I was impatient to know whether Harry had succeeded in liberating the Time-Turner.

I spotted the grin that passed between Harry and Ron as Ron changed the subject, now discussing something stupid about Quidditch practice. They knew I was dying to find out if Harry had the Time-Turner and were purposely withholding the information to annoy and frustrate me — and it was definitely working. Determined not to let them beat me, I listened to them talk until Ron finally told Harry to show me before I exploded.

With a laugh, Harry pulled the Time-Turner out of his pocket and held it up. The tiny glass phial containing the golden sand glinted as the light caught it. I let out a small squeal of joy and hugged Harry tightly before hitting him hard on the arm and admonishing him for winding me up. Ron gave a gruff laugh so I cuffed him, too, laughing in return at his cry of foul play.

I was sure now that my plan would work. We were meant to get the Time-Turner. We were meant to go back in time and change the past. Perhaps the Headmaster and Professor Snape had even left hints that I had subconsciously picked up on, knowing that I would piece them together and come up with a plan. That was why Professor Dumbledore hadn't returned the Time-Turner to the Ministry of Magic, I was certain. He knew we would need it.

Yes, I know, I can see you shaking your head and thinking, is she mad? Thinking she could just waltz back in time and change it without consequences, especially when Professor Croaker's Law says that the furthest a person can travel back without serious chance of injury or of harming time itself is only five hours not almost twenty years.

Of course, you would be absolutely right. But I was filled with the passionate zeal of someone on a quest who knows their path is right and true, even if it appears to others to be completely crazy — and worse still, I had managed to convince my best friends that I was right. There was no one to make us stop and think about the possible consequences of our actions if we got it wrong. The whole thing about stepping on a butterfly and causing the entire world to change was pushed to the back of our minds in the excitement of how we were going to rid the world of Voldemort.

I spent every spare minute in the Library — not exactly unusual for me, I know, but I was studying old Arithmancy books and making calculations so I could work out exactly how far I had to go back.

In preparation for the trip the three of us had visited the Room of Requirement, now a storage facility similar to the one in which Harry had placed the Half-Blood Prince's Potions book after his catastrophic and almost deadly fight with Draco Malfoy a few weeks earlier. There we left a stash of things including Harry's Invisibility Cloak, which I could hopefully retrieve once I reached my destination if I required them.

I had no idea how long I would be staying in 1978, but the uniform robes hadn't changed and blue jeans and a plain t-shirt were always acceptable Muggle attire at any time; it wasn't as if I was going to be attending a ball or anything. I was going to get in, assess the situation, do what needed to be done to change the future and get back to my own time as soon as possible.

Harry and Ron were going to cover for me. I would pretend to be ill with a miscellaneous bug and feign the need to stay in bed. They would tell my dorm mates that I had requested to be left alone to sleep it off and to make sure I had closed the curtains around my bed knowing that no one was likely to disturb me. After all, I was only going to be gone for a few minutes at the most at this end, however long it took at the other.

During dinner, I made my way downstairs and waited in the dusky twilight for Harry and Ron to join me outside the castle. I was impressed that Ron had thoughtfully wrapped some food in napkins for me to eat later as I was absolutely starving. It was a small gesture, but one so unusual for him that for a moment I couldn't help but wonder whether what I was going to do would change my friendship with Harry and Ron in some way. But there was no time to waste on that sort of thinking. We still had the plan to run through one more time and it was too late to start doubting it now.

So, trusting that I had done my calculations correctly, and holding tightly onto a suitcase containing a few clothes and some of my more important books, I hugged first Harry and then, for slightly longer, Ron and with a smile, I set the Time-Turner that was hanging around my neck.