'A year ago today, my sister was murdered. At least, that's what I hoped had happened.

'See, this is the way I remember it. After my sister, Sara, died, the woman who killed her was going to be sent to prison, but I agreed for us both to participate in the Empathy Program. The idea was that they would copy my memories of Sara, and the pain I felt when she died, and put them into the mind of her killer, a woman called Julia. They would leave them there for a month, during which time Julia wouldn't even know that she was the killer. Then, when Julia was given back the memory of what she'd done, she'd be so genuinely remorseful that she would be considered safe to rejoin society. That's what was meant to happen.'

'But it didn't?'

'No.

'The Empathy program was deemed immoral, and shut down. I guess now I can see why. Julia didn't have her memories of Sara removed, and she got to carry on living a free life, completely unaware that she was the murderer. That's how I remember the situation.

'Problem is, memories aren't nearly as reliable as they used to be. I remember it happening like that, but the way Julia remembers it, I was the one who killed Sara, who she thinks was her own sister.

'Now don't get me wrong. I loved Sara, and I would have gladly given my life to save her. But when it came down to it, I couldn't be certain that it wasn't me that had killed her. For all I knew, she wasn't even my sister, and everything I remembered was a lie.'

'But why today? You've lived with this doubt for a year. What made you decide you needed to see Julia?'

'It was only a tiny thing.

'I came downstairs for breakfast yesterday morning, and when I turned on the TV they were showing a report about that suspected murder. That woman, Rachel Taylor, who fell off of the top of that building, you know the one? Oh, sorry, of course you do. I'm getting mixed up. It's been a long day.

'They were showing these photographs of the woman, kind of a memorial thing. Something about her, maybe her eyes, reminded me of Sara, and I couldn't bury my feelings any more. I couldn't keep living with the possibility that I might have killed her. I had to know for sure, so I went to see Julia.

'At first, we managed to keep it civil, which was impressive, considering that each of us, to the other, represented everything we hated. She had set me out a mug of tea, and we agreed that we would be entirely honest with one another, while we tried to work out which one of us had killed the person who we both loved most in the world.

'It was harder than you would have thought. Neither of us had any living relatives, nor were we particularly social people. Sara had lived on the other side of town, so neither of our neighbours were going to be able to remember her either.

'To be honest, it didn't matter that much to me by that point, because as soon as I'd seen Julia, I felt a massive weight lift from my shoulders. It wasn't obvious, but she had an aura about her, as if, if I got on her nerves, she would drop her middle-class, polite facade and turn on me. I was completely convinced that she was the sort of person who was capable of murder. Well, I was right, of course. You could not imagine how happy I was when she held that knife to my throat.

'I thought I was going to die, but I'd be dying a happier man than I'd been for the last year, safe in the knowledge that I hadn't killed Sara.'

'You didn't die though, did you?'

'No. I didn't. That's when you broke down the door, with all the other police officers.'

'And why were we there?'

'You were there to arrest Julia.'

'That's not all, Steve. Why else were we there?'

'For me. You were there to arrest Julia and me. You were there to arrest Julia and me, because we were the prime suspects in the murder of Rachel Taylor.'

'And why were you the prime suspects?'

'We were the prime suspects, because, as you told us, a year ago today, me and Julia killed Rachel's sister. Me and Julia killed Sara Taylor, and Rachel agreed to let us go free, on the condition that we participated in the Empathy Program. We were given Rachel's memories of Sara, and we were forced to forget our own parts in the murder.'

'Did you kill Rachel Taylor, Steve?'

'No. No, I didn't. Honestly, I think she probably committed suicide. It's the anniversary of Sara's death, and believe me when I tell you that memories of her haunt me every waking moment. Rachel has the exact same memories. It's not hard to imagine that it got too much for her.'

'And did you kill Sara Taylor?'

'Yes. I think I probably did.'

'Why did you kill her?'

Steve looked around the interrogation room, and sighed before replying.

'See, that's the bit that gets me. I genuinely have no idea.'