I do what I always do. I step off the Way and set about finding Siggy, to engineer our chance meeting.
       The cars on this London's roads are mostly Volkswagens, noisier than back home, and flimsy-looking. They pump thick clots of diesel smoke across the pavement. Police walk in pairs, black uniforms, guns at their hips. People in the street are thin. Grey clothes hang from shoulders like wire coat hangers.
       I find him behind the counter of a coffee shop in the centre of town, close to Goering Platz, which I am more used to knowing as Trafalgar Square.
       "Just a coffee," I say. Sigmund Fifteen slops some oily-looking liquid into a mug and pushes it across the counter. He shows no sign of recognition. Naturally enough, since he's never seen me before.
       "Wow!" I say. The hot liquid tastes of chicory and old tyres. "What kind of coffee is this?"
       He shrugs. "Just the normal shit."
       From a seat in the corner, I study him as unobtrusively as I can. He looks ten years older than I remember. He frequently tilts his head and looks furtively around, as if fearing a blow. I remember other times with him. Drinking champagne on the deck of a boat as sunlight throws diamonds onto water. A moonlit walk home from the pub, our breath crystallising in front of our faces.
       None of that could happen here. The newspapers are full of news from Europe, mostly Germany. So far as I can work out, this is a world where Hitler won the war. That should be weird and intriguing. Except to people who live here; for them I guess it's normal, if a bit of a bummer.
       I pay for my coffee and leave without speaking again to Siggy. It's like seeing someone you love after they have suffered a long illness. I walk around for a long time, blinking away tears.
       The moon at last heaves itself up above soot-stained rooftops. There is nothing here for me in this version of London. Hitler, if he's still alive, can have it.
       I move on.

       After another dead end with sixteen, Sigmund number seventeen is promising. He's living in the same London flat, and he steps into the pub one evening when I'm alone at a corner table with a glass of white wine. I'm reading a book I'm sure he'll know.
       "How far in are you?" he says, having sidled up to my table in what he thinks is a subtle manner.
       "A few pages. Should I keep going?"
       "I never advise a girl to back out. Buy you a drink?"
       When you've seduced the same person a dozen times, it's easy. The hard bit is keeping a check on my feelings when we go to his flat. He is having sex with a virtual stranger, but for me it's different. The familiar smell and feel of him, the way he moans when I push against him, these things make me dizzy. I've missed him so much, but I can't show it. After all, we've never met.

       I first met him in Cornwall five years ago. He had been drifting round Europe for years, never staying anywhere long. His parents died when he was young and left him enough money to live as he liked.
       I was stupid to show him the Way.
       It was a party near Falmouth. A big house on a hill above the estuary, like something out of Daphne du Maurier. Music poured from open windows onto couples dancing on a wide lawn, beneath lights hung from trees. We were drunk, and I led him down the garden and into a meadow above the sea. The moon was full, laying a shimmering trail across the Atlantic.
       "I can show you something amazing," I said.
       "Right here?" I think he expected me to take my clothes off.
       "It's only possible at a full moon. Close your eyes." I had guessed early on that he would be able to see the Way. Not everyone can. "Tell me what you feel under your feet." I walked him backwards.
       "Grass," he said. "Grass again. Wait --."
       "Open your eyes."
       His back foot rested on a thin strip of light a couple of inches above the grass. The luminous path snaked away from us, rippling along the cliff top, to disappear over a low rise.
       "What is it?"
       "It's called the Way. Not everyone can find it."
       "But what is it?"
       "Try it and find out. Don't go far, just a few minutes and then come back."
       That first time he did as I said. I let him go and he disappeared into the invisible space the Way occupies between worlds. I waited alone in the moonlit field for ten minutes until he flickered back into view. The Way winked into darkness behind him.
       "Oh. My. God," he said, when he reappeared three feet in front of me. "It was the same place, but completely different. No party, no people. The house was dark and empty. Like I travelled in time and came back."
       "Not time. It's weirder than that. It's like a path to different realities, versions of the world in which different choices were made, other decisions taken."
       "How many different worlds?"
        "Infinite, in theory. How many actions are taken every day that could have gone differently? There are places where we didn't meet, places where we met in different ways, at different times. Go far enough and there will be worlds where your parents didn't meet and you were never born."
       "Like that movie Sliding Doors? Could I have ended up in a world where you were Gwyneth Paltrow?"
       "Is that what you'd like?"
       "No." He hugged me. "I like what's right here."
       Which was what I wanted, right? I knew from the start that he was a wanderer. I wanted to impress him and give him a reason to stick around. If I showed him something no one else could offer, he would want to stay with me. I tried to stop him searching for lollipops by giving him the keys to the sweetshop.
       "When can I go again?"

       Six months later, I'm in London. I take a train out to the south suburbs and walk to the middle of a small park at midnight. I drink from a half-pint bottle of vodka as I sit beneath an oak tree. You need to relax for the Way to appear, getting as close as possible to a trance. It comes easily to me now. Alcohol helps.
       I'm dozing off when I become conscious of the world around me growing brighter. I open my eyes to find the silvery path a few yards in front of me, drawing a line across the park as if the moon rolled past before it climbed into the night sky. I shoulder my rucksack and step onto the Way. The path beneath my feet becomes brighter, and the park beyond fades behind a wall of fog. I count paces, and after thirty I step off the path into the darkness of an empty meadow.
       Next morning, I book into a cheap guesthouse and look for Siggy. I know enough from our time together to find his London flat. He doesn't go there much, just pitstops between his trips abroad, so finding him takes a bit of hanging around.
       While I wait, I have time to observe this version of London, from my vantage point of coffee shops and park benches. The clothes, the buildings, the buses, the banknotes--everything is the same. Sometimes it's like that. If you don't go too far down the Way, the divergence can be hard to spot.
       He turns up on the fifth day, and I contrive an accidental first meeting by walking into him as he leaves the flat, dropping my rucksack.
       "Hey, I'm sorry." He picks it up and hands it back with a smile.
       "It's me. I'm so clumsy."
       Ten minutes later we are in a café, and he's telling me about his childhood. I've heard it before, of course. We swap phone numbers and things progress as you would expect, although it isn't as romantic in London as it was on the Cornish coast. Nevertheless, Siggy puts off his next planned trip, to north Africa, and when I drop heavy hints about the insecurity of my accommodation, he invites me to move in with him.
       We spend huge amounts of time in bed. When we emerge, we do all the things that young lovers do in London; walk hand in hand on riverside walks, visit museums, eat out in a dozen different restaurants. We spend every day together, which is gorgeous at first, but I should know it isn't sustainable.
       A few weeks in, I notice Siggy occasionally spaces out while I'm talking, his eyes on something far away. After we've been together a month, he takes to going out alone for walks early in the morning. He says it helps him get his thoughts clear before the day gets busy.
       He's getting restless. I should have anticipated it. Every girl he's ever known, he's left behind. He never settles; what's round the next corner is always more interesting than what's in front of him. This was why I let the original Siggy in on the secret of the Way. I wanted him to share something with me that he couldn't have with anyone else. What could be more appealing to a wanderer like him than a pathway to limitless worlds?
       This time it will be different. I'll keep him firmly in this world. I cook us fantastic meals. Domestic goddess: tick. I surprise him by initiating spontaneous sex in the park on the way home from the pub. Pixie dream girl: tick. I back off and leave him space to himself. Understanding life partner: tick.
       One morning, I find him in the kitchen, eating toast and drinking coffee. His rucksack, packed and ready to go, is by the door. "I need to go away for a while." He avoids my eyes. "I meant to mention it."
       "Where? How long?"
       "You can stay here while I'm gone. Just until…"
       Wait, I want to say. If you stay I can show you something much more exciting than wherever you're going.
       "I'll see you," he says as we briefly kiss at the door.
       I leave before he gets back. Showing him the Way won't keep him. If he's willing to leave me to visit Estonia or Spain, or wherever he's going, how will I get him back from the America where John Kennedy wasn't shot, the Europe conquered by the Incas, or the million other worlds he could reach along the Way?
       I'll try again. There are plenty of other Siggys out there.

       The original Siggy moved in with me the week after I first showed him the Way. I had a cottage near the village of Helford, beside the estuary that ran into Falmouth Bay. It was a small place, only two rooms upstairs. At nights, we made love in my squeaky bed and lay afterwards with the breeze from the window cooling our tangled bodies. We walked together along the shore, or inland onto the moor. We took turns to cook in the evening, juggling pans in my tiny kitchen.
       It scared me how quickly I knew that he was the man I had been waiting for, the one I wanted to be with for the rest of my life.
       He wanted to know all about the Way. How did it work? How come nobody else knew about it? How did I know about it?
       "It's weird," I said. "No one showed it to me, it was always kind of there when I was a child, on the edge of things. It took me ages to understand that most people couldn't see it. I was fifteen when I first experimented with travelling along it."
       "Wasn't it scary finding all that out by yourself?"
       "It never occurred to me at the time. It was only later I realized the risks."
       "How does it work?"
       "I don't know much about physics. Some scientists believe there are multiple universes. After all, there are particles--quarks or quorns or whatever--that appear and disappear, going someplace we can't detect. If particles, why not people?"
       "Yeah, but a ghostly path and a full moon? That's a fairy story, not science."
       "Maybe in other places it isn't a path," I said. "Maybe it's a door, or a machine, or a magic carpet."
       "Or a flying sleigh, drawn by reindeer."
       "Why not? Most of what we perceive is conditioned by the limitations of our senses, or the concepts in our heads. Some people can see a shining path, some can't. Maybe others see a mirror, stepping through it like Alice."

       I want to tell this Siggy everything I've done since I lost him; places I've been, people I've met, and the weird twists that history makes. The different versions of him that I've loved and lost. Of course, I can't tell him any of that. Everything must be new between us.
       We're in London, so there is plenty for us to do together in the early stages; museums, theatre, restaurants. I ensure there are gaps between our dates. Other times, I spent too much time with him too early, and the relationship burned too fierce and too brief. I'm not making that mistake again.
       Late one evening, back in his flat, I notice a couple of new books on his bedside table; a history of Persia and a guide to modern Iran.
       "Planning a trip?"
       "Nothing definite." He doesn't meet my eyes as he answers. Warning sign: Sigmund Wanderlust returning.
       "I know a trip you'd really like. Somewhere hardly anyone else has ever been."
       "Yeah? What's that?"
       "We need a full moon."

       It was bollocks, of course, the story I told the first Siggy, about how I discovered the Way all by myself in my innocent youth. Siggy, for all that he was lovable and sexy and clever, was a man, and he swallowed my lies without chewing.
       I didn't tell him about my teacher: Miss Bell, the mysterious and mixed-up Vanessa Bell who turned up at our small girls' boarding school in the Somerset hills the September I was seventeen. Miss Bell spent a lot of time in her room, or else you'd see her heading off for solitary hill walks. She gave very little of herself away and avoided meeting your eyes when she talked. But I noticed that her eyes often strayed lower down, and more than once I saw her strolling past the netball court, where we ran around in our navy-blue knickers.
       One night, I stayed awake past midnight and crept along to her room in my thin nightie. I knocked on her door three times before she opened it.
       "Ellie, what is it?"
       "Oh, Miss Bell, I'm so upset and I don't know who to talk to."
       It's easy to turn on the tears if you've worked yourself up enough. At my school, most girls cried at least once a day, so we were never out of practice.
       "It's very late," poor Vanessa Bell said. She looked both ways along the corridor and back to the front of my nightie, which I had neglected to button properly. "You'd better come in."
       There wasn't much in her room; a portable gas fire that made a comforting sighing sound, piles of books against the walls, and a threadbare sofa. And a bed. Once inside, I kept crying, and Vanessa comforted me. After a short time, there was more comforting than crying and eventually, when we moved from her couch to her bed, we were both wholly engaged in comforting. Even at that age I was certain I preferred boys, but Vanessa Bell knew what she was doing and we both enjoyed it.
       Not content with introducing me to sex, Vanessa also showed me the Way. Imagine my surprise! I knew the mysterious Miss Bell had secrets, but this was far beyond anything I could have guessed. She never told me how she found it, but she was eager to show me her secret. I suspect she thought we might use it to run away together. Instead, I used it to run away from her.
       I wondered if she was sad when I went. I thought about that a lot. Later, when Siggy left me.

       "Forty-six, forty-seven. Keep your eyes closed. Forty-eight. Steady, no cheating. Forty-nine, Fifty. Brilliant. Right on the money."
       Two people cannot travel together on the Way. But you can separately travel the same distance. It took me a long time, and some messy experiments, to work that out. If I had known it when I first met Siggy, think of the trouble it would have saved me. Nowadays, I'm keen to avoid my old mistakes, so before I let Sigmund Seventeen on the path, I insist we practice pacing out exact distances. We rehearse until we can walk fifty steps with our eyes closed and cover exactly the same distance.
       Full moon. I put him on the path first and tell him to go twenty paces. I follow, and walk the same. I walk down a tunnel of coiling mist. When I stop, I stare for a long time at the tunnel wall. The hairs on the back of my wrists stand erect. I find I can sense the right place, so long as I get close enough. Maybe it's something to do with the years I've spent on the Way. Or the time I've spent searching for Siggy. Either way, there's a tingling sense of rightness when I'm stepping off at the same place he did.
       I walk through the misty tunnel wall and my feet sink into the sand of a moonlit beach. At first, I fear I'm alone, but then I see Siggy a few yards away, at the water's edge, picking up small pebbles and skimming them across the breakers.
       It seems I have finally done it. I have Siggy, and we can travel the Way together. I can keep him.

       This London looks like the dustmen collect rubbish from the rest of England and dump it in the streets. Some of the heaps of rubbish move, and when I look closer I see people in there--dozens of them in rags, sleeping in the open. Many have ugly sores on their faces, limbs missing, skin the colour of margarine. There are no buildings less than twenty years old and the roads are thick with bicycles and wheezing buses.
       Why is this England so dismal? It seems to have taken an alternative path in the 1950s. War in Korea. America and Russia standing behind their local players, angling for advantage. Bluff and counter-bluff, just like the history I was familiar with, but in this world, someone fucked up and conflict spread. Luckily, intercontinental missiles were primitive and rare, but bombers still got through, leaving radioactive craters where Boston, San Diego, and Leningrad used to be. London was spared, but bombs on Birmingham and Plymouth fractured Britain in a way that has taken decades to recover from.
       Sigmund number ten works in a bookshop on Piccadilly and it's easy to engineer our usual spontaneous meeting.
       "Catch 22. I love this book," he says as he rings up my purchases. "And Waterland. It's like you've picked all my favourites."
       "Really? I was just browsing. You don't believe in fate, do you?"
       He buys me a coffee during his break. Despite the unpromising world around him, this Siggy is close to the way I know him; sexy and intelligent, full of good humour. He hasn't had the opportunities to travel enjoyed by other editions of him, with their trust funds and early orphanhood, but on our first date he tells me about camping trips he's made, hitching through Europe alone.
       Our month together is not quite the perfection of the first time--it never is--but he is sweet and tender and it's a wrench to show him the Way. I know by now the risk I run in laying this gift at his feet. What decides me is the world around us. There's no way I'm staying here for the long term, so my only chance of keeping Siggy is to take him somewhere else.
       "Have you ever seen a movie called Sliding Doors?" I say one day, casual as you like.
       Amazingly enough, I succeed in getting him through to my own home reality, with me close behind. We have another gorgeous two months together in my cottage, in between showing Siggy the wonders of a world where people are twice as wealthy and no one dies of radiation sickness. But this isn't the Sigmund I fell in love with. Silences blossom between us all too soon, and he does what is in his nature and, on the third full moon, he disappears along the Way without me.
       I try not to get depressed, but this is pretty much what happens every time, whether I show him the Way or not. If Siggy is close enough to the man I first met for me to want to be with him, he is also inclined to hit the road when the initial thrill passes. If he isn't close enough, I do one myself.

       That first time--with my first Sigmund, the real one--the next full moon came on a rainy night. We packed a rucksack with food and a flask of vodka and hiked up to a wooded hill overlooking the estuary.
       "You don't need to rush this," I said. We walked along a narrow road between dripping trees. "There's always next month."
       "You're kidding. I've waited a month for this."
       "You always want to look over the next hill."
       "It's quite a hill. Don't you find it exciting?"
       "You could stay with me."
       "I am staying with you. I just want to see more of what's out there."
       We sheltered under an oak tree on the crest of the hill. I spread a blanket on a relatively dry patch, and we drank vodka as rain drummed on the leaves above us.
       "We could just stay here and have sex."
       "Come on. You've done this more than me. Let me have another try."
       We stood close together again and I made him close his eyes. The Way appeared behind him, tracing a pale line across the ground, running west from us, clinging to the edge of the trees.
       "Don't go too far."
       "Can't we go together?"
       "I don't think it works that way." If only I'd known back then. I kissed him. "Come back soon."
       "I will." He stepped onto the luminous path and disappeared.
       I never saw him again.

       This time I stay a month and travel to London. Armed police are on the streets and there are metal grilles on the windows of public buildings. No one meets your eye in railway stations and other public places.
       I take a casual job in a supermarket in south London. The hours are variable and I have plenty of time to search for this world's Sigmund. By now, I know I'm not going to find the one I lost. He could be marooned far from home, perhaps unable to find the Way again. Or perhaps he's in a version of the world so close to our own, apart from lacking a version of him, that he doesn't realize his mistake. Is he somewhere with another version of me, unaware of the difference? Or searching in the wrong place, wondering why I left without him? Somewhere out in the Multiverse, flicking through the infinite deck of cards looking for the queen of hearts.
       I eat in a pub and watch the television news. There is street fighting in Belfast, with soldiers firing live rounds at masked rioters. One day, the news is dominated by a fire at an Irish Internment camp at Feltham, near London. The newsreader says it was started by rioting inmates, celebrating the 30th anniversary of Margaret Thatcher's death from an IRA bomb in her Brighton hotel.
       I find Sigmund Thirteen by tracing his parents. Thatcher's death was a mixed blessing for them. After the prime minister's assassination, years of upheaval and civil unrest changed things enough that they didn't die in the car crash that orphaned my Siggy so young. On the other hand, the main reason for them avoiding the car crash was the year they spent in an internment camp, along with thousands of other Irish-born Londoners. His mother came out in decent shape, but his father was badly beaten, and emerged in poor health, deaf in one ear and able to walk only with a stick. They lost their money in a fruitless legal action seeking compensation, and Siggy grew up less wealthy, less independent, and worse educated than the man I knew.
       I introduce myself by bumping into him in a Stockwell pub, spilling my own drink down my blouse. One thing leads to another, and we have fun for a few weeks. I can see at once that it won't go the distance. He's not the same; he has suffered enough knocks, and lived in an unfriendly enough London, that he lacks the free spirit I loved in him. He's less quick to laugh, less excited about new experiences. He doesn't close his eyes when I kiss him; he watches over my shoulder as if afraid someone will attack while his guard is down.
       I could just enjoy a few weeks with him before slipping away. But I feel sorry for him, and decide to show him the Way. I don't know whether he returns to the dismal world he comes from. I don't wait to find out. As soon as he winks out of sight on the path, I retrieve the rucksack I hid earlier, and take my own leave. I never go back.

       Once I master the trick of using the Way with Siggy, it feels like everything is right at last. For a long time, over a year, we travel together through a bewildering series of alternative Englands. A country hardly out of feudalism, where Elizabeth I never ruled and the Roman church continued to reign. A land of scattered villages inhabited by short farmers, on the edge of a Europe where Mongol invaders went undefeated centuries before. A low-rise London of public baths, grand boulevards and pavement cafes, crammed with wealthy travelers from all over Europe, mostly speaking what sounds like a variant of Latin.
       Admitted, our time together isn't quite the thrill of that first month, with the first Siggy, but maybe that would have become less exciting too, over time. We just never had the time. I don't worry that we argue at times, and the physical side of the relationship cools off. That's natural. We can still make it work.
       Eighteen months later, on our last trip, I step off the Way seconds after Siggy. We are on a four-lane highway. One carriageway is empty, except for large cracks in the asphalt, through which trees have thrust themselves. The other carriageway is a wall of metal; abandoned, rusty cars, nose to tail as far as we can see. Behind the wheel of the nearest car is a skeleton, dressed in faded scraps of clothing. The sky is clear as cut glass, with a scent of long-dead fires on the warm breeze.
       We spend a long day hiking back along the line of cars to see what they were fleeing. We see a few people, but only ever at a distance. They are thin and they run away when they see us. Other than that, we find only streets strewn with rubbish and bones, and looted supermarkets, carpeted with broken glass. Something killed most of the human population, and it happened quickly. A plague, swine flu, anthrax--who knows?
       There are thousands of birds, filling the world with birdsong louder than I have ever heard. They give me the creeps. When they look at us, in their eyes is a hint of a species memory that we are food, if only we would lie still long enough.
       Neither of us speaks for a while after we return, walking through the woods under the full moon. I break the silence.
       "That's one set of sliding doors I don't want to go through again."
       "What do you mean?"
       "Sliding Doors? The movie with Gwyneth Paltrow."
       "I don't know it."
       "You do. You always used to mention it whenever--." I bite my lip.
       "Whenever what?" He stares at me.
       "Nothing."
       I can see the incoming tide of realization flow into his face, as he mentally joins up the dots of our time together; the things I've said, the way I always know what he wants. He knows what the Way can do, so of course he can work it out.
       "It wasn't me who said that, was it?"
       "Probably not. My mistake. Shall we get home?"
       "Was it someone who looked like me?"
       "I'm not talking about this now."
       I go straight to bed when we get home, leaving Siggy watching a late movie. Maybe he found Sliding Doors. I wake up in the small hours and he is kneeling beside the bed.
       "How many times?" he says, when I sit up and switch on the light.
       "What do you mean?"
       "You know what I mean. Tell me how many times we've met before. Or you've met versions of me."
       "Can you make me a cup of tea?"
       I know I'm busted, and when he returns with the hot drink I tell him everything. Well, a lot of it. It's enough.
       "I can't believe this."
       "You know how the Way works. It isn't so hard to imagine."
       "I don't mean I doubt it's possible. I'm just amazed you would do it. You've got this amazing gift. A secret most people would kill for. And this is what you do with it?"
       "I don't have to justify it."
       "It's like you're in a tall, beautiful building. There's a breathtaking view across a lake to mountains beyond. The world outside is full of wonder. The building itself is stuffed with miracles--astonishing people, rooms full of treasure."
       "Very poetic."
       "But what do you do? You're in windowless room, sitting by the photocopier. You're making a copy of a photo of a man you once knew. Then you make a copy of the copy and so on until the image is so blurred it isn't clear if it's the same man."
       "It isn't like that."
       "How can you live this life?"
       "I love you."
       "Don't be stupid! You loved someone once, not me. Why not look for him?"
       "I tried."
       "Keep trying. You might not find him, but at least I can understand you trying, and maybe you'll find a worthwhile life along the way. Instead of this dead-end you've put yourself in."
       He stands, and picks up the rucksack that had been on the floor, hidden behind his body.
       "I'm leaving."
       "No surprise. It's what you do."
       "It's what I'm doing. Right here, right now. I can't answer for the other guys. They're your problem."
       He leaves.

       Seventeen didn't understand. All that crap about photocopying photocopies. I admit, it may be a tiny bit less special each time, but it's worth it. The first few weeks with Siggy are always worth it. It's a shame it always goes wrong after that, but maybe it won't go wrong forever.
       Anyway, a copy of a copy of a copy can still be read, if the print is big enough. And the print between me and Siggy was big.
       I'm upset for a while, but I get over it. Full moon again and I'm back in the woods, stepping onto the Way. I'm not yet sure yet how to play it with Sigmund Eighteen, when I find him.
       But I will find him. And this time I'll get it right.