When I was eight my sister saw something that she wasn't supposed to, though I didn't realise what it was at the time. There's always something, isn't there? Walking in on the baby sitter being groped by her boyfriend and standing there looking confused, that kind of thing. It's the right age for getting into trouble without realising it. My problem is what happened afterwards made me forget to try and figure it out. It didn't seem important after that and she never mentioned it again. That's the bit I really don't like.

It started when she stole my bike. We had been playing out in the street and I'd been annoying the hell out of her. I say street, but it wasn't really. That's just the easiest way to describe it. We were on holiday. My parents had taken us to a cabin in the Highlands by this loch, and the nearest town was about half an hour away. The street we were on was parallel to the water and pretty much deserted apart from the occasional car. It was awesome.

I remember it was about sunset. Mum had let us go off on our own because, let's face it, there wasn't any mischief we could get into. We were both strong swimmers and the locals were more scared of us than anything. The two of us were like a flying tumble-weed of energy, almost inverted copies of each other – my darker hair the shadow to her light. It always was like that.

We're twins, though I'm a little older and always make sure she remembers it. At eight I wished she was a boy, so we'd be identical. You can't play pranks if your twin is a girl, and her blonde hair always did get her more attention. Besides that, girls were – well, they liked stupid stuff like ballet and ponies and anything pink. Even her bike was pink. That really did appal me. I'd been taunting her about it – that's why she stole mine.

I raced after her along the road. I knew she was going out to this ruined fort just around the peninsula. It was some kind of ancient settlement - great for hide-and-seek. I'd called her a wimp because something about the place really freaked her out. Last time we were there I had amused myself trying to creep up on her and make her scream. Being an older brother has its benefits, even if it is only by a matter of half an hour, and she didn't like it one bit. She was determined to prove she was braver than me. As far as twins are concerned half an hour might as well be a whole year.

I skidded her bike to a stop by the edge of the road where she had dumped mine. I was frowning because there was a big rubbish truck parked just by the track up to the fort but there was no one else around. There were no houses in the area, and dustmen always come in the morning. I knew that because my mother used to wake me up with her rushing round the house every Wednesday when she forgot to put the bins out the night before. I scrambled up the path to the top of the hill, trying to catch up with my sister. Something wasn't right.

I scrambled over a wall, dropping down over the other side out of her sight and yelled out for her to wait for me. I doubled my speed, hopping over another wall and down through a half destroyed doorway, my feet thudding on the hollow-sounding turf. From up there you could see almost the whole loch. It was turned a dark silver against the burnt orange sun in the sky. I stopped a moment, wondering why the clouds were so purple when they were meant to be grey or white. I scratched at my sunburnt nose and scrabbled over another wall. As I stood on top of it and turned back, I saw the top of my sister's head dipping up and down as she went over the uneven terrain. I grinned, the uneasiness leaving me. She had come up here first and I still managed to be ahead of her! That's girls for you.

I dropped down and pressed my back against the wall. I planned to double back towards her and jump out just in front of her to scare her. I dropped to my knees and crawled along the bottom of the wall, quickly turning the corner of the house, or whatever it had once been, before she could catch me up and see what I was doing. It was like a rabbit warren up there.

I snuck along a little more, feeling like a secret agent or an explorer. I heard her call out to me, wondering where I was. I stayed quiet. Moving a little further along, I noticed that someone had been hanging about by the walls of the ruins. There were a series of scuffed footprints on the muddy ground and a few cigarette butts. Some of the stones were loose as I put my hand on them for balance. I hurried up. There wasn't usually anyone about up here. It wasn't the kind of place for a convenient smoke, but I didn't think of that then.

I rounded the corner, coming out at a level area matted in dense low lying grass. I ran quickly across it. Each step made an echoing thud, interrupting the still evening. I scanned around for my sister and as I looked towards the peak of the hill I found myself staring at a man with a cigarette hanging from his lips. Something about his stare struck a shiver through me. He didn't see me – he was looking at something else and there was a twisted snarl on his face. My clumsy feet stumbled. I tripped violently, falling to the ground. I didn't make a sound though the palms of my hands were stinging. I knew I shouldn't let him know I was there.

I rolled over and lay there a moment, searching for whatever I had tripped over. Mt eyes focused on a mound of damp fur and I moved closer. It was a cat, and it was dead. It didn't move when I reached out and prodded it. At first that didn't strike me as odd, God knows why. Its eyes were forced open in a static stare. They made my skin crawl, but it was the smell that got to me. I dragged myself up before my stomach heaved. The man had disappeared from view. Being eight, I bent down to look at the cat again, ghoulish curiosity getting the better of me. It looked a bit like the stray we had been feeding scraps. There was a line of red along the animal's belly – staining its orange fur red. It looked a lot like it had been cut open.

It was about then that I heard my sister scream.

The sound brought me to my senses and my automatic honing device took over. Somehow I know that if she's in trouble I will always be able to find her. Probably I shouldn't say know – it's nothing more than a feeling. True or not I ran in the direction of the sound, following my instincts. I know the intonations of her voice like I know those of my own, and her scream was more than just afraid.

I rounded the corner of the ruin and caught a glimpse of her. She was standing there - her wide eyed gaze fixed on something I couldn't see. Her face was startlingly pale in the twilight. She seemed frozen to the spot. I ran faster, jumping down towards her from the cliff level I was at, curving round above her. She screamed again and it died away, sounding like her voice had given up on her – died in her throat.

For some reason I glanced up at the small cliff I had just left. Maybe I heard a noise, or maybe it was intuition. The man – the one with the cigarette and the scowl – was standing there, looking down at her. I don't know why, but his stare was angry and it made my slowing legs move. He wanted to hurt her – I've never known anything so certainly. She didn't notice me until I was nearly next to her. If only she had moved, things would be different now.

I looked up. I knew what was happening like you know the second before you fall over that you just lost your balance. I looked up and I saw rocks. They were falling towards her - except I knew that falling was the wrong word. They had not just fallen. I didn't stop to shout at her, to warn her - I just jumped at her, trying to push her out of the way.

I was too late to move myself. I guess I'd known that before I jumped, but when things happen so quickly you don't really think at all. I took her place and the last thing I felt was a sudden shock of hot pain as a heavy boulder struck me on the side of the head. I was vaguely aware of her falling under me as the darkness descended. The man left us for dead. That much I know.

That was the last time I ever saw my sister.