The kitchen door was unlocked and I stepped out into the breezeless evening, everything green and pink and gold. Though I wasn’t aware of any human presence in the garden, some indefinable disturbance of the mood encouraged me to set off down the path towards the playhouse at the bottom. It was only a few months old then, a cute little thing with a ladder to the roof and a slide curving around the side, constructed and customized by Bram. The door, usually swinging open, was closed.

I could hear all the typical sounds of the street’s gardens on a summer evening – husbands and wives summoning each other for dinner, last calls for children’s bedtimes, dogs and foxes and birds and cats objecting to one another’s proximity – but I did not add to them by calling Bram’s name because I was by now certain he was in the playhouse.

What was I expecting as I stepped over the lip of the slide and peered through the window? A crack pipe? An open laptop with the frozen image of something unspeakable? In all honesty, I expected to find him sneaking a cigarette and I was already calming down, planning a retreat. There were worse crimes, after all, and I wasn’t his GP.

A second passed when the shapes were too abstract to identify, but only a short one because the rhythm was real enough, even banal: a man and a woman having sex. A married man and a woman who was not his wife having frantic sex because time was of the essence here. Yes, she was out for the evening, but, still, there were kids in the house, he couldn’t have them waking up and finding the place abandoned. Telling Mum all about the scare in the morning in that breathless way of theirs, competing to make the most dramatic claim: ‘The whole house was totally empty!’ ‘We thought Daddy had been murdered!’

There was a horrible chewing sensation in my gut as I stood there, overwhelmed by an unexpected sense of power. Should I fling open the door, as he deserved, or should I creep away and bide my time? (For what purpose? To see if he would do it again? This, surely, was proof enough that he would.) Then I caught a glimpse of his face, the sickening, feral grimace of excitement, and I knew I had no choice. I pushed open the door, watched them startle like animals. A half-full wine glass set to the left of the door wobbled but did not fall.

‘Fi!’ Bram mouthed, breathless, dazed.

You know, a year or so ago, I overheard my sister Polly talking to a friend of hers about me: ‘It’s like she’s a normal intelligent person in every other way, but she has this blind spot when it comes to Bram. She’ll forgive him anything.’ And I’d wanted to storm in and tell her, ‘Once, Polly! He did it once!’

Well, now it was twice. And I mean it when I say it was a relief to discover it, a relief so powerful it was almost pleasure.

‘Bram,’ I replied.

Bram, Word document

I’ll kick off with the thing in the playhouse, which I have no doubt is where Fi would begin, even though it’s a red herring, I can tell you that for nothing. But it was the official catalyst, our assassination of Archduke Franz Ferdinand, and so it has its place in this story, I accept that.

The name of my partner-in-crime is beside the point, and since I doubt her husband knows what she’s been up to and won’t relish her being associated with me and my crimes, I’ll call her Constance in this document, after Lady Chatterley. (You’ll allow that little joke, I hope. And no, I’m not a great reader of the classics. I saw the movie once – Fi’s choice.)

‘I thought I’d drop by,’ she said that evening at the door, with the unmistakable air of goods being offered. She seemed very drunk, but it could have been the exhilaration of being the initiator, an aphrodisiac in itself, as men have known for millennia. ‘You said you’d show me the inside of your playhouse, remember?’

‘Did I? I’m not sure there’s anything to see,’ I said, grinning.

She waggled her iPhone. ‘Can I take a photo to show my carpenter?’

‘My carpenter?’ I teased. ‘Well, you can, yes, but you do know you can just buy these things flat-packed at B&Q? All I did was fix it together and then build the slide.’

‘But the slide is the best bit,’ she exclaimed. ‘Maybe I’ll try it out – if my bum doesn’t get stuck.’

What was that if not an invitation to look?

She was wearing a white cotton dress, puffed at the shoulders and gathered under her breasts with a tie, the fabric so light it caught on her thighs every time she took a step.

‘Any chance of a glass of wine?’ she asked as we passed through the kitchen.

You know, it’s not true that in moments of sexual temptation men degenerate into lower mammals, all rational thought obliterated. It’s more a weakening by degrees. First, when I noticed the dress riding up, I thought, Don’t even think about it. No way. Then, when I was opening the wine, I thought, Well, you had to crack some time. Soon after, as I was leading her down the garden path (that sounds bad), I thought, Come on, at least not here, not with your children sleeping inside. Then: All right, just this one time and then never again.

By which time we were inside the playhouse, door closed, and she was pressing the full length of herself against me: her body was overheated, her hair humid, her face on fire. It was the heat that did it, not the softness or pertness or wetness, not the scent of sweat or Chanel or wine. There’s such an urgency to hot skin, the nearness of the other person’s blood, your own responds as if it’s magnetized.