I don't remember the exact day Colin (not his real name) became part of my life. A fleetingly glimpsed neighbour I'd sometimes nod to, I knew he was a long-distance truck driver and I think he knew who I was. When our paths crossed, he would seldom meet my gaze. I don't even remember when I first heard his name. A familiar voice uttered it, though: my wife's. It wasn't by way of an introduction, although years later I did wonder how that might have gone. "Honey, you've seen that handsome man with the blond hair, broad shoulders and light tan who lives at number 18? His name's Colin." But no. Rather more mundanely, she referred to him matter-of-factly in conversation. "Colin took the remains of that old fence to the dump for me today, honey." Or, "Oh, by the way, Colin mended the lawn mower. Then he mowed the lawn."

Colin was indeed the helpful friend who lived down the road. It is easier now to see that he was a more astute people-watcher than he seemed. He must have known that my job often took me abroad, and he probably had me profiled as a workaholic, as insensitive to my wife's needs as I was trusting of her fidelity. By then we'd been married for some time and had two young children. While I was away, my wife stayed at home taking care of the kids. I thought we were happy but, in truth, I was too wrapped up in my job to know. Life went on. And secretly, day by day, it was written down: my wife was a meticulous diarist and spent 20 minutes every evening logging the day's events.

One afternoon I walked into our bedroom and noticed she'd left her diary on the bed. This was unusual but, stranger still, it was open. I went to put it in her bedside drawer, but as I closed the pages I caught sight of the word "Colin". My immediate impulse was to slam the diary shut and try to forget I'd seen his name before it had a chance to sink in.

Instead, I started to read. The entries stretched back months, detailing their covert liaisons - romantic, practical, but mostly sexual. The descriptions ranged from the relatively tame ("Kissed and cuddled today, it was lovely") to the kind of things you get in the racier passages of a Mills & Boon novel - nothing too graphic, but surprisingly comprehensive. My jaw ached with panic and I felt the sudden flush of adrenaline.

Of course, I confronted her. I wanted to yell at her, but my initial anger was quickly anaesthetised by shock. I felt numb, confused. With tears in her eyes, she said she hadn't been happy for years and that Colin provided an escape. At that moment, I didn't know what to say. It was four or five hours before we could sit down and talk. We discussed the usual options, including divorce, but decided to stay together for the sake of the children, make a fresh start. Next day, she told Colin it was over.

We didn't see him for a couple of weeks after that - he'd been driving his lorry on the continent. But Colin never did return. The news that he had gone missing on a ship, presumed lost overboard, was broken to us by his next-door neighbour. My wife's first reaction was stunned disbelief, as was mine. Then she turned away and covered her mouth, trying to stifle any sobs. Thoughts and emotions more tangled than ever, I tried to comfort her.

Colin's death was confirmed by the positive identification of a body washed up on the beach. Some weeks later, my wife asked if we could drive to the crematorium so she could lay some flowers and say her final farewells. It felt strange but, in the hope of her finding some kind of closure, I told myself it was the right thing to do.

Somehow, and completely irrationally, I felt involved in Colin's death, as if a personal timeline had been derailed on my behalf and destiny rewritten to rescue a failing relationship. Slowly we tried to put it behind us and his name was never mentioned again. A few years later we had another child and our marriage entered a new, happier phase. I vowed to be a more attentive husband and adjust my work-life balance. But I couldn't forget the affair, especially how close it had happened to home.

I should have trusted my instinct: 12 years later, my wife ran off with my best friend.

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