No one can know.

That was the first thing Blair had said to him when it was over, shattering though the haze of warmth and well-being and...something else...he had been feeling.

Yet he wasn't surprised by the words.

Because what had happened was so unexpected, so shocking, so surreal that it would have been possibly to find the words to express it, even if he had wanted to tell someone.

They had been hanging out over the Christmas holidays and it had been much more fun than he expected. But she had still been sarcastic, and he had still been cabbage-patch, and it hadn't been like things had changed between them, not really.

More like they had finally accepted something that had already been there. That they had finally acknowledged that at some point over the past three years they had stopped hating each other and had become friends, like it or not.

Then she had decided that it would be sad and pathetic for them not to celebrate New Years' just because everyone else had abandoned them. There had been alcohol, and then suddenly there had been something else.

One moment they had been laughing hysterically over something that would not have been all that funny if they hadn't been drunk, then they had been kissing each other. And neither of them had wondered if it was the right thing to do, or how she could have so lowered her class standards, or how he could have so lowered his moral ones. They had only wondered how they could have managed to go twenty years without being kissed like this.

And then the kissing had quickly become something else. And it, too, had been like nothing else.

But of course no one could know. Who would even believe it, when they could barely believe it themselves?

It should have felt sordid. The brief clinch, Blair's caustic words, the hurried dressing afterwards. It would have done, if Blair hadn't squeezed his hand before he left.

And he had squeezed back. Surprised at how easily he understood the wordless communication. With the passing of that slight pressure between their hands, each had acknowledged something they could not admit out loud. That they didn't regret it.

The knowledge seemed to ignite something in him.

And he realised that Blair had said no one could know. Not that it could never happen again.

It was what she should have said, Blair reflected. She should have made it clear that for someone like him to get to sleep with someone like her had been a one in a million chance, the result of a strange confluence of circumstances. A twist of fate, like winning the lottery. Something it would be impossible to have happen to the same person a second time.

But she hadn't been able to say any of that. Because the whole thing had been so unanticipated, and when it had explodedbetween them, it had been completely beyond her power to control.

It wasn't fireworks. When she had had those with Chuck they had lit up her world with colour and sparkle. But then they had faded, leaving only black, bleak night. This was a forest fire, erupting from a small spark to a riot of irrepressible, all-consuming heat in a matter of seconds.

And in the weeks that followed, it continued a steady burn.

They would meet somewhere, and be having a perfectly friendly, ordinary conversation, then she would be crawling onto his lap or he would be dragging her to a deserted hallway.

They never discussed it, even though it was usually in both their natures to compulsively talk out everything. They chatted about other things, conversed about almost every other topic, as their friendship continued to grow.

It was as if they were superstitious that if they were talk about this amazing thing they had discovered, either to each other or anyone else, it would disappear.

She tried to end it when Serena finally blew back into town, having secured Ben's release.

She ignored his calls, and avoided being alone with him at social functions, shaking off his touch, and his attempts to get her on her own.

'We both knew this wasn't forever,' she finally hisses one day. 'Go be with Serena.'

He does go to Serena. Ever since she had come home she had continued to hint that she would not be averse to trying to again, had invited him had for cosy coffees and long lunches. She had seemed confused when he turned down most of her invitations.

Now he heads straights to her. And tells her that while she will always have a special place in his heart, he knows now that they could never have a future together.

He goes back to Blair and this time doesn't listen to her harsh words, her attempts to shut him out. It is the first time they spend the whole night together in the same bed.

They still don't tell anyone, though it soon becomes obvious to absolutely everyone.

They sneak off together too often, muttering lame excuses. Blair leaves from a group brunch, saying she has an appointment to get her perfectly-manicured nails done. Dan gets up thirty seconds later and says he has a 'thing' to go to. Chuck is belligerent, Serena is resigned, Jenny and Vanessa appalled, Nate and Eric madly amused.

But it's not the patent sexual tension that really gives them away, reveals to everyone that this thing – whatever it is – is serious. It's the easy camaraderie the two share whenever their together.

When Dan's first book appears a short while later, a spectacular success, especially for one so young. No one is surprised that the heroine is a mouthy but romantic brunette who manages to get underneath the hero's skin in spite of himself.

When the movie rights to Dan's book are picked up Blair is the natural choice for leading lady. Blair and Dan decide that it will be the perfect time to finally come clean on their return from L.A. They tell everyone that while working together their friendship blossomed into something more.

But of course, everyone knows.