I WALKED BACK TO THE HOUSE where Kathy’s lover lived. I stood there for an hour, watching. Eventually the door opened, and he emerged. I watched him leave. Where was he going? To meet Kathy? I hesitated, but decided not to follow him. Instead I stayed watching the house.

I watched his wife through the windows. As I watched, I felt increasingly sure I had to do something to help her. She was me, and I was her: we were two innocent victims, deceived and betrayed. She believed this man loved her—but he didn’t.

Perhaps I was wrong, assuming she knew nothing about the affair? Perhaps she did know. Perhaps they enjoyed a sexually open relationship and she was equally promiscuous? But somehow I didn’t think so. She looked innocent, as I had once looked. It was my duty to enlighten her. I could reveal the truth about the man she was living with, whose bed she shared. I had no choice. I had to help her.

Over the next few days, I kept returning. One day, she left the house and went for a walk. I followed her, keeping my distance. I was worried she saw me at one point, but even if she did, I was just a stranger to her. For the moment.

I went away and made a couple of purchases. I came back again. I stood across the road, watching the house. I saw her again, standing by the window.

I didn’t have a plan, as such, just a vague, unformed idea of what I needed to accomplish. Rather like an inexperienced artist, I knew the result I wanted—without knowing quite how to achieve it. I waited awhile, then walked up to the house. I tried the gate—it was unlocked. It swung open

and I stepped into the garden. I felt a sudden rush of adrenaline. An illicit thrill at being an intruder on someone else’s property.

Then I saw the back door opening. I looked for somewhere to hide. I noticed the little summerhouse across the grass. I raced silently across the lawn and slipped inside. I stood there for a second, catching my breath. My heart was pounding. Had she seen me? I heard her footsteps approaching. Too late to back out now. I reached into my back pocket and took out the black balaclava I’d bought. I pulled it over my head. I put on a pair of gloves.

She walked in. She was on the phone: “Okay, darling. I’ll see you at eight. Yes … I love you too.”

She ended the call and switched on an electric fan. She stood in front of the fan, her hair blowing in the breeze. She picked up a paintbrush and approached a canvas on an easel. She stood with her back to me. Then she caught sight of my reflection in the window. I think she saw my knife first. She stiffened and slowly turned around. Her eyes were wide with fear. We stared at each other in silence.

This was the first time I came face-to-face with Alicia Berenson. The rest, as they say, is history.