The other archaeologists got bored and stopped paying attention, and Macker gave Mark's back the finger and swaggered back to his patch as if all eyes were still on him. I was suddenly, fiercely glad that I would never in my life have to be a teenager again. I ground out my cigarette on a stone and was buttoning my coat and turning to go back to my car when the realization slammed home in the pit of my stomach (sucker punch, wicked drop through black ice): the trowel.

I stood very still for a long time. I could feel my heartbeat, quick and shallow, at the base of my throat. At last I finished buttoning my coat, found Sean among the huddled army jackets and picked my way across the dig towards him. I felt curiously light-headed, as if my feet were paddling effortlessly a foot or two above the ground. The archaeologists threw small swift glances at me as I passed: not inimical, exactly, but perfectly, studiedly blank.

Sean was troweling earth away from a patch of stones. He had headphones on under his black woolly hat, and he was bobbing his head gently in time to the tinny bam bam bam of heavy metal. "Sean," I said. My voice sounded like it was coming from somewhere behind my ears.

He didn't hear me, but when I took a step closer my shadow fell across him, faint in the gray light, and he looked up. He fumbled in his pocket, switched off the Walkman and pulled the headphones down.

"Sean," I said, "I need to talk to you." Mark whipped round, stared, then shook his head furiously and attacked the bank again.

I brought Sean out to the road. He hauled himself onto the hood of the Land Rover and pulled a greasy doughnut wrapped in plastic out of his jacket. "What's the story?" he asked sociably.

"Do you remember, the day after Katharine Devlin's body was found, my partner and I brought Mark in for questioning?" I said. I was impressed with how calm my voice sounded, how easy and casual, as if this were only a small thing, after all. It becomes second nature, interrogation; it seeps into your blood until, no matter how stunned or exhausted or excited you are, this remains unchanged: the polite professional tone, the clean, relentless march as each answer unfolds into question after new question. "Not long after we brought him back to the site, you were complaining that you couldn't find your trowel."

"Yeah," he said, through a huge mouthful. "Hey, it's OK if I eat this, right? I'm starving, and Hitler will have a total cow if I eat during work."

"That's fine," I said. "Did you ever find your trowel?"

Sean shook his head. "I had to buy a new one. Bastards."

"OK, think carefully," I said. "When was the last time you saw it?"

"Finds shed," he said promptly. "When I found that coin. Are you going to, like, arrest someone for stealing it?"

"Not exactly. What's this about a coin?"

"I found this coin," he explained, helpfully. "Everyone was all excited and stuff, because it looked old and we've only found like ten coins on the whole dig. I took it to the finds shed to show Dr. Hunt-on my trowel, because if you touch old coins the oils in your hands could fuck them up or something-and he got all excited and started getting out all these books to try and ID it, and then it was half past five and we went home, and I forgot my trowel on the table in the finds shed. I went back to get it the next morning, but it wasn't there."

"And that was the Thursday," I said, my heart slowly sinking. "The day we came to talk to Mark." It had been a long shot, anyway, and I was surprised at how badly let down I was. I felt idiotic and very, very tired; I wanted to go home and go to sleep.

Sean shook his head and licked grains of sugar off his filthy fingers. "Nah, before that," he said, and I felt my heart rate start to pick up again. "I sort of forgot about it for a while, because I didn't need it-we were mattocking back that fucking drainage ditch-and I figured someone had, like, picked it up for me and forgot to give it back. That day you guys came for Mark, that was the first time I needed it, but everyone was going, 'No, I haven't seen it, uh-uh, wasn't me.'"

"It's identifiable, then? Anyone who saw it would know it was yours?"

"Totally. It's got my initials on the handle." He took another enormous bite of doughnut. "I burned them on ages ago," he said, in muffled tones, "this one time when it was lashing rain and we had to stay inside for, like, hours. I have this Swiss Army knife, see, and I heated up the corkscrew with my lighter-"

"At the time you accused Macker of taking it. Why?"

He shrugged. "I don't know, because he does dumb shit like that. Nobody was gonna steal it steal it, not with my initials on it, so I figured someone had just taken it to piss me off."

"And do you still think it was him?"

"Nah. I only realized after, Dr. Hunt locked the finds shed when we left, and Macker doesn't have a key-" Suddenly his eyes lit up. "Hey, was it the murder weapon? Shit!"

"No," I said. "What day did you find the coin, can you remember?"

Sean looked disappointed, but he thought about it, staring into space and swinging his legs. "The corpse showed up on a Wednesday, right?" he said eventually. He had finished his doughnut; he balled up the plastic wrap, tossed it in the air and swatted it into the undergrowth. "OK, so it wasn't the day before that, because we were doing the fucking drainage ditch. The day before that. Monday."

I still think about this conversation with Sean. There is something oddly comforting in the memory, even though it carries its own inexorable undercurrent of grief. I suppose that day was, though it still comes hard to acknowledge this, the pinnacle of my career. I am not proud of a lot of the decisions I made in the course of Operation Vestal; but that morning, at least, in spite of everything that had come before and regardless of anything that came after, that morning I did everything right, as surely and easily as if I had never put a foot wrong in my life.

"Are you sure?" I asked.

"Yeah, I guess. Ask Dr. Hunt; he logged it in the finds book. Am I, like, a witness? Am I gonna have to testify in court?"

"Quite possibly," I said. Adrenaline had burned off the fatigue and my mind was speeding, throwing out permutations and possibilities like a kaleidoscope. "I'll let you know."

"All right," Sean said happily. Apparently this made up for the murder-weapon disappointment. "Do I get witness protection?"

"No," I said, "but I do need you to do something for me. I want you to go back to work and tell the others that we were talking about a stranger you saw hanging around a few days before the murder. I was asking you for a more detailed description. Can you pull that off?" No evidence and no backup: I didn't want to spook anyone, not yet.

"Course," Sean assured me, offended. "Undercover work. Excellent."

"Thanks," I said. "I'll get back to you later." He slid down from the hood and bounced off towards the others, rubbing the back of his head through the woolly hat. He still had sugar around the corners of his mouth.

I checked with Hunt, who went through his logbook and confirmed what Sean had said: he had found the coin on the Monday, a few hours before Katy died. "Wonderful find," Hunt told me, "wonderful. Took us quite a long time to…um…identify it, you know. No coin specialist on site; I'm medieval, myself."

"Who has a key to the finds shed?" I asked.

"Edward VI base penny, early 1550s," he said. "Oh…the finds shed? But why?"

"Yes, the finds shed. I've been told it's locked at night. Is that correct?"

"Yes, yes, every night. Mostly pottery, of course, but you never know."

"And who has a key?"