PROLOGUE

The trouble with sleeping with someone is that you can never react to them the same way again.

Now when he walks into my lab I go hot all over. I actually start sweating. My chest throbs with how hard and fast my heart is pounding. And I feel weak and watery at the knees, although that might just be the guilt.

He, of course, maintains an imperious indifference to my presence.

I keep looking at him, even though this makes it worse. Sometimes he catches my eye and looks right back, his trademark hard stare, and I try to work out if it Means Something - that is, if it means something more than For God's Sake Molly Stop Staring People Will Notice.

I worry about John in particular. I mean, this business with Sherlock... John kind of owns Sherlock. He's possessive about him. Well, so would I be. So I am, in fact, now that this has happened. But John's had Sherlock a long time. Plus, of course, John and I have been an item through all this upheaval. It would probably seem pretty tactless of me if John found out what happened between me and Sherlock.

I think Sherlock thinks this too. He is certainly jumpy around John when I'm in the room.

But is it cheating, when your boyfriend was only a substitute for the person you've cheated on him with?

I suspect it probably is.

I think John will be upset when he finds out.

OK. I know he will. And I feel awful about that. But this thing, this Sherlock thing, well it has been a long time coming. No pun intended. Surely John knows that. Surely he will understand.

What a fool I am. Of course he won't understand. He will be hurt and angry and he will have every right to be.

And standing here, up to my elbows in the deceased, with Sherlock's pale blue eyes fixed on me like he finds my work fascinating, captivating, erotic, like he is thinking at Hadron-Collider speed how he can get me back into bed to show me a few new things he has casually learned over years of having at all times the most magnetic personality in the room – I find I don't care about John's feelings. Not one bit.

That makes me a bad person.

But Sherlock is still staring at me, his eyes widening and his nostrils flaring, and I still really, really don't care.

SIX WEEKS EARLIER

Hi Molly. Fancy a pint? My treat. Just want a chat with someone who doesn't think he was an arse. J.

I stared at the text for a long time, my thumb held carefully away from the screen so I was in no danger of typing a reply before I had fully thought things through.

A drink with John. A chat. About Sherlock, obviously. Such a terrible idea in lots of ways. John was unhappy, depressed, obsessing over why Sherlock had done it. John needed to move on and couldn't. Sitting around talking about it with me would not help him forget Sherlock.

And I was not going to be any good at cheering him up or at reminiscing about the good old days. Firstly although I am a reasonable liar, the lie I was keeping for Sherlock was so huge that it threatened to burst free at any moment. It bubbled up to my lips like a foul belch after too many lagers on a first date, ruining your chance of a goodnight kiss with the bloke you have allowed to get you drunk...

Secondly, those good old days. They never existed. My relationship with Sherlock was one of subservience and invisibility. He would show up, all arrogance and control, and lay out what he expected to happen. I made it happen. End of. He barely acknowledged me. I made a play for him, a couple of times – who wouldn't? – but he just let my unsubtle advances slide off him like a duck dodging raindrops. Him and The Work. Also, him and John. Sherlock only had room for one human at a time and from the moment he walked into my lab, John Watson was that human.

Everyone assumed it was a gay thing. This was mainly because of Sherlock. His total indifference to girls. His total indifference to everybody, really, but also the way he would deny girlfriends but not deny that he and John were an item. He was only doing it to wind John up, but because it worked so well, he kept doing it until their coupledom was established as a widely believed fact.

So between The Work, capital letters, God he was pretentious, and John, I was just a convenient nonentity with the ability to wheel in a trolley containing something Sherlock wanted to take a look at.

And as for John - Nice enough, but dull. Solid. Reliable. All those things which don't make your pulse quicken.

A bit old for me, too, to be honest. I mean. Nice blue eyes. In good shape for his age. But no spark.

Still, a drink. It wasn't like he was hitting on me. He wanted a hand to hold (another total turnoff) and a bit of company.

"Anything interesting?" asked my friend Harpinder after I'd been goggling at this text for five minutes.

"Not really," I said. "What do you think about this new shift pattern they're threatening?"

"It's crap," she pronounced. "Everybody loses. I don't know what they were thinking."

"Massive efficiency savings, that's what," I told her. "If we can all use off peak travel it cuts our outgoings and gives them a reason to refuse a salary review next year."

"If just seems really unfair on people," Harpinder said. "I mean, some of us have lives."

She clapped her hand to her mouth. "Oh god," she said. "Sorry. I didn't mean -"

"It's all right," I said with a nonchalant air. "I know I don't usually provide much of the gossip round here. But as it happens I do have something going on tonight "

"Ooh, get you! A date?"

"Yup," I lied, and with that, John's fate was sealed.