Probably you could say I came back fighting because I had been suckered, almost fatally, and this was my last chance to make up for that; or because the only way I would ever get my career back-It’s my job, I had said to Daniel, before I knew the words were going to come out-was if I got a solve here; or because our lost Operation Vestal had poisoned the air around me, and I needed an antidote. Maybe a little of all three. But this was the one I couldn’t get away from: no matter what this woman had been or done, we had been built into each other since we were born. We had led each other to this life, this place. I knew things about her that no one else knew, in all the world. I couldn’t leave her now. There was no one else to look through her eyes and read her mind, trace the silvery lines of runes she had left trailing behind her, tell the only story she had ever finished.

All I knew was that I needed the end of that story, that I needed to be the one who brought it home, and that I was frightened. I don’t scare easy, but just like Daniel, I’ve always known that there’s a price to pay. What Daniel didn’t know, or didn’t mention, is what I said right at the beginning: the price is a wildfire shape-changing thing, and you’re not always the one who chooses, you’re not always allowed to know in advance, what it’s going to be.

The other thing hitting me over and over, with a horrible sick lurch every time: this could have been why she had come looking for me, this could have been what she had wanted all along. Someone to change places with her. Someone longing for the chance to toss away her own battered life, let it evaporate like morning mist over grass; someone who would gladly fade to a scent of bluebells and a green shoot, while this girl strengthened and bloomed and turned solid again, and lived.

I think it was only in that moment I believed she was dead, this girl I had never seen alive. I’ll never be free of her. I wear her face; as I get older it’ll stay her changing mirror, the one glimpse of all the ages she never had. I lived her life, for a few strange bright weeks; her blood went into making me what I am, the same way it went to make the bluebells and the hawthorn tree. But when I had the chance to take that final step over the border, lie down with Daniel among the ivy leaves and the sound of water, let go of my own life with all its scars and all its wreckage and start new, I turned it down.

The air was so still. Any minute now, I would have to go back to Whitethorn House and do my best to wreck it.

Out of nowhere I wanted to talk to Sam so badly it was like being hit in the stomach. It felt like the most urgent thing in the world, to tell him, before it was too late, that I was coming home; that, in the ways that mattered most, I was already back; that I was scared, terrified as a kid in the dark, and that I needed to hear his voice.

His phone was off. All I got was the voice-mail woman telling me, archly, to leave a message. Sam was working: taking his turn surveilling Naylor’s house, going through statement sheets for the dozenth time in case he had missed something. If I’d been the crying type, I would have cried then.

Before I understood that I was doing it, I set my phone number to Private and dialed Rob’s mobile. I pressed my free hand flat over the mike and felt my heart going slow and hard under my palm. I knew this was very possibly the stupidest thing I’d done in my life, but I didn’t know how not to do it.

“Ryan,” he said on the second ring, wide awake; Rob always had trouble sleeping. When I couldn’t answer, he said, with a sudden new alertness in his voice, “Hello?”

I hung up. In the second before my thumb hit the button I thought I heard him say, fast and urgent, “Cassie?” but my hand was already moving and it was too late for me to pull it back even if I had wanted to. I slid down the side of the tree and sat there, with my arms wrapped tight around myself, for a long time.

There was this night, during our last case. At three in the morning I got on my Vespa and went down to the crime scene to pick Rob up. On the way back the roads were all ours, that late, and I was going fast; Rob leaned into the turns with me and the bike barely seemed to feel the extra weight. Two high beams came at us around a bend, brilliant and growing till they filled the whole road: a lorry, half over the center line and coming straight for us, but the bike swayed out of the way light as a stalk of grass and the lorry was past in a great whack of wind and dazzle. Rob’s hands on my waist shook every now and then, a quick violent tremor, and I was thinking of home and warmth and whether I had anything in the fridge.

Neither of us knew it, but we were speeding through the last few hours we had. I leaned on that friendship loose and unthinking as if it were a wall six foot thick, but less than a day later it started to crumble and avalanche and there was nothing in the world I could do to hold it together. In the nights afterwards I used to wake up with my mind full of those headlights, brighter and deeper than the sun. I saw them again behind my eyelids in that dark lane, and I understood then that I could have just kept driving. I could have been like Lexie. I could have hit full speed and taken us soaring up off the road, into the vast silence at the heart of those lights and out on the other side where nothing could touch us, ever.

It only took Daniel a couple of hours to come up with his next move. I was sitting up in bed, staring at the Brothers Grimm and reading the same sentence over and over without taking in a word of it, when there was a quick, discreet rap on my door.

"Come in,” I called.

Daniel put his head in the door. He was still dressed, spotless in his white shirt and shining shoes. “Do you have a minute?” he inquired politely.

“Of course,” I said, just as politely, putting down the book. There was no way this was a surrender or even a truce, but I couldn’t think of anything either of us could try, not without the others there for weapons.

“I just wanted,” Daniel said, turning to close the door behind him, “a quick word with you. In private.”

My body thought faster than my mind. In that second when his back was to me, before I knew why I was doing it, I grabbed the mike wire through my pajama top, gave it a hard upwards yank and felt the pop as the jack came free. By the time he looked around again, my hands were lying innocently on the book. “About what?” I asked.

“There are a few things,” Daniel said, smoothing the bottom of the duvet and sitting down, “that have been bothering me.”

“Oh?”

“Yes. Almost since you… well, let’s say arrived. Small inconsistencies, growing more troubling as time went on. By the time you asked for more onions, the other evening, I had serious questions.”

He left a polite pause, in case I wanted to contribute anything to the conversation. I stared at him. I couldn’t believe I hadn’t seen this one coming.

“And then, of course,” he said, when it was obvious that I wasn’t going to answer, “we come to last night. As you may or may not know, on a few occasions you and I-or, at any rate, Lexie and I-have… Well, suffice it to say that a kiss can be as individual and unmistakable as a laugh. When we kissed, last night, it left me more or less positive that you’re not Lexie.”

He gazed at me blandly, across the bed. He was burning me all over again, every way he knew how: with my boss, with the boyfriend he’d guessed at, with the brass who would not approve of an undercover smooching a suspect. They were his brand-new remote-controlled weapons. If that mike had been plugged in, I would have been a few hours away from a grim trip home and a one-way ticket to a desk in Offaly.

“Absurd though this may sound,” Daniel said tranquilly, “I’d like to see this supposed stab wound. Simply to reassure myself that you’re actually who you’re claiming to be.”