I had been inside the house for just over two hours when there was a knock at the door.

The girl was a little taller than me; maybe five foot six or seven. She wore a white T-shirt and blue jeans that emphasised how enviably skinny she was, but behind her dark sunglasses I could tell that she had been crying. Her hair was white-blonde and cut in impressive zigzags to her sharp, angular chin, and she was chewing gum ferociously. Despite her composure, I could tell that she was extremely nervous, and I had the awful sensation that I knew why.

“Are you Elizabeth Matthews?” she asked, although it was more of a statement than a question. The slight quiver in her voice betrayed that she was about to cry, but I felt no sympathy for her.

I shifted my weight nervously from foot to foot, like a bird on a telephone wire. What did this girl want? “Yes,” I said, hoping that she would not notice my indecision.

“You’re Sean’s girlfriend, right?”

Again, there was no question about it. I merely nodded. I was Elizabeth Matthews, Sean’s girlfriend, and she was going to say her piece whether I wanted her to or not.

“Well,” she continued, and there was a hint of triumph, of new arrogance in her demeanour. She removed her sunglasses, and I could see cold blue irises, and the redness that betrayed her earlier tears. “I have something to tell you.”

I leaned against the door frame. “Oh.”

This was not the reaction she had hoped for. It threw her off her stride, and silenced her momentarily. “I’m Sarah,” she said, “and Sean and I have been going out for over a year.” She looked me in the eye and whispered, “He’s cheating on you.”

I bit my lip hard. Sarah stared at me with undisguised pity. I didn’t care. Let her think that I was devastated. Let her think that this was something new. However, as she moved to touch my arm sympathetically, I flinched away from her, and crushed her, the way you crush an insect: with a quick, sudden blow that they’ll never see coming, except I used words instead of the back of my hand:

“You think you’re the first person to say that, don’t you?”

Sarah looked stunned. “What?”

“He told you that you were something special, didn’t he? That he’d never normally cheat on his girlfriend, but, that you...you were too special for that. That he didn’t want to hurt her, but you -- you made it impossible; you made it so that there was only one thing he could do, and it killed him, but he did it anyway, because you were worth it. He told you that the two of you were in love.”

I spoke swiftly and bitterly, as if spitting poison over her, but keeping eye contact for as long as I could. Every word was fluid and confident. The range of emotions that Sarah exhibited was incredible: first, there was the denial, the unwillingness to believe; then came the fury -- the self-loathing, and the hatred that she reserved for me; and then, finally, the grief, the realisation that everything you had been told was a lie, the unquestionable knowledge. When I had finished, she seemed to have wilted, withered, all of the self-assurance and certainty gone.

“I don’t believe you,” she whispered, but she did; there was no doubt about it, she believed me.

“I’m sorry,” I said: empty, hollow words. “You’re not the first.”

Her face crumpled. “What am I supposed to do?”

I was gentler now. “You’re a pretty girl, Sarah. You can have anyone you want. Just cut off all ties with him. Don’t answer the phone; don’t answer his texts, cut him out of your life. You don’t need him.”

Sarah began to nod. “You’re right,” she murmured, through tears, “you’re right.”

And then she was gone. Crying quietly, holding her sunglasses in one shaking hand, she was gone, and I knew she’d never come back.

Feeling shaken, I retreated indoors, letting my eyes travel over the framed photographs on the mantelpiece: Liz and I on her last birthday, arms thrown over one another’s shoulders. The words ‘I Heart S.D.’ were just visible, scrawled on the back of her hand with a felt-tip pen.

S.D.: Sean Danvers.

Lizzie Thorpe: my life-long best friend; the girl I was house-sitting for, who now, thanks to me, would never have to know the truth.