It started with simple mathematics. There were two seats left on the bus and two people, a man and a woman, at the stop. So one would end up sitting next to me. The man was dressed in ragged clothes and looked like he might smell, possibly of piss. She was a divine vision in sensible grey. I crossed my fingers and repeated to myself, tall thin blonde, tall thin blonde - As if it mattered who I spent the next twenty tedious minutes of my life sitting next to.

I looked up as she slipped gracefully into the seat next to me. I inched to one side, hardly moving at all. Excellent. "Here, let me make some room."

"You can see me. I wasn't sure if you'd be able to."

"Um." Sometimes "um" is all you can say.

"And hear me, obviously."

"Do I know you?"

"No, but you will. Or rather, you might."

The conversation was disconcerting but also promising. Who can resist a mysterious woman? "Pleased to meet you - " I held my hand out. She hesitated and stared at it for several seconds as if it was a difficult decision. Then she shook it. Her fingers were like velvet-covered ice.

"Sharrie," she said. "God, I can even touch you. We don't have much time, Chris. I have to explain."

"How did you - ?"

She looked directly into my eyes for the first time. I hadn't fully appreciated the colour green until that moment. "I don't even know if I can make you believe." I realised that she wasn't looking at me now but beyond me at her own reflection in the window. "I'm surprised about my face. I expected it to be all smashed up from the accident."

"Accident?"

"Yes. We'd been out to celebrate our wedding anniversary. It was one of those stupid things that always happen to other people. We were going too fast, skidded on the ice, off the road and into a tree. I was killed instantly. You died on the way to hospital."

"I did?" I didn't see the joke but would have laughed anyway if she hadn't looked so close to tears. "When?"

"February fourteenth, 2003."

You know that feeling you get in your stomach when a beautiful woman you've never met tells you she's your wife's ghost and you will die together in a car crash in exactly five years time? No? There was only one way I could reply. "Um."

"You once told me that time only came into existence at the Big Bang and everything that ever will happen happened at that moment." It sounded like the kind of stupid thing I'd say to try to seem deeper and cleverer than I really am. "I don't want that to be true. I want you to have a second chance. Think of anything that's ever happened to you, good or bad. Snap! Snap! Snap! Life is full of coincidences. We make split second decisions all the time but we have to make them with too little information. I want to give you a chance to have that information and make the right decision. Not one based on a stupid coincidence of two people sitting next to each other on a bus."

"How did you come back? Can the past be changed?"

"I don't have a rule book." She shook her head and looked at me with those wonderful eyes for the last time. I could see that I had disappointed her somehow. "These aren't the questions you should be asking, Chris."

"What are?"

"You should be asking 'Do we love each other? Are we happy? Is our life worth dying for?'" The bus is slowing down. "This is where I get off. And on."

It's simple maths again. Two people get off. One gets on. Now there are two empty seats and the new passenger isn't a stranger. She was visibly younger and her hair and clothes were completely different but it was her, of course. Sharrie. When I saw her I realised I had believed every single crazy word.

And suddenly I had to make one of those split second decisions. I could live five years with Sharrie or forty, fifty with someone else. Maybe. Someone better. Maybe. I didn't have enough information. I didn't know then that if I caught that stupid bus again and again a thousand times, day after wretched day for five desperate years she'll never ever get on it again. I just knew that if I moved my briefcase onto the seat next to me she would sit somewhere else and we would live different, longer lives. Then, before I knew it, it was all over. I did. She did. And we will.

Five years ago. Today. And there hasn't been a day, an hour, a minute since that I haven't regretted it.