“I would’ve decided exactly the same way if you’d been standing right here,” I said. “I’m a big girl, Sam. I don’t need protecting.”

“I don’t like that fella,” Sam said sharply. “I don’t like the way he thinks and I don’t like the way he works.”

I slammed the oven door. “He’s trying to solve this case. Maybe you don’t agree with the way he’s doing it-”

Sam shoved hair out of his eyes, hard, with his forearm. “No,” he said. “No, he’s not. It’s not about solving the case. That fella Mackey-this case has bugger-all to do with him, no more than any other murder I’ve worked, and I didn’t see him showing up on those ones pulling strings right and left to get in on the action. He’s here for the crack, so he is. He thinks it’ll be a great laugh-throwing you into the middle of a bunch of murder suspects, just because he can, and then waiting to see what happens. The man’s bloody mad.”

I pulled plates out of the cupboard. “So what if he is? All I’m doing is going to a meeting. What’s the huge big deal?”

“That mentaller’s using you, is the big deal. You’ve not been yourself since that business last year-”

The words sent something straight through me, a swift vicious jolt like the shock from an electric fence. I whipped round on him, forgetting all about dinner; all I wanted to do with the plates was throw them at Sam’s head. “Oh, no. Don’t, Sam. Don’t bring that into this.”

“It’s already in it. Your man Mackey took one look at you and he knew something was up, figured he’d have no problem pushing you into going along with his mad idea-”

The possessiveness of him, standing in the middle of my floor with his feet planted and his fists jammed furiously in his pockets: my case, my woman. I banged the plates down on the counter. “I don’t give a flying fuck what he figured, he’s not pushing me into anything. This has nothing to do with what Frank wants-it’s got nothing to do with Frank, full stop. Sure, he tried to bulldoze me. I told him to fuck off.”

“You’re doing exactly what he asks you to. How the hell is that telling him to fuck off?”

For a crazy second I wondered if he could actually be jealous of Frank and, if he was, what the hell I was supposed to do about it. “And if I don’t go to the meeting, I’ll be doing exactly what you ask me to. Would that mean I’m letting you push me around? I decided I wanted to go tomorrow. You think I’m not able to do that all by myself? Jesus Christ, Sam, last year didn’t lobotomize me!”

“That’s not what I said. I’m just saying you haven’t been yourself since-”

“This is myself, Sam. Take a good look: this is my fucking self. I did undercover years before Operation Vestal ever came along. So leave that out of it.”

We stared at each other. After a moment Sam said, quietly, “Yeah. Yeah, I suppose you did.”

He dropped down on the sofa and ran his hands over his face. All of a sudden he looked wrecked, and the thought of what his day had been like sent a pang through me. “Sorry,” he said. “For bringing that up.”

“I’m not trying to get into an argument,” I said. My knees were shaking and I had no idea how we had ended up fighting about this, when we were basically on the same side. “Just… leave it, OK? Please, Sam. I’m asking you.”

“Cassie,” Sam said. His round, pleasant face had a look of anguish that didn’t belong there. “I can’t do this. What if… God. What if something happens to you? On my case, that had nothing to do with you. Because I couldn’t bloody well get my man. I can’t live with that. I can’t.”

He sounded breathless, winded. I didn’t know whether to hold him tight or kick him. “What makes you think this has nothing to do with me?” I demanded. “This girl is my double, Sam. This girl was going around wearing my fucking face. How do you know your guy got the right one? Think about it. A postgrad who spends her time reading Charlotte bloody Brontë, or a detective who’s put dozens of people away: who’s more likely to have someone out to kill her?”

There was a silence. Sam had worked on Operation Vestal, too. Both of us knew at least one person who would happily have had me killed without a second thought, and who was well able to get the job done. I could feel my heart banging, hard and high under my ribs.

Sam said, “Are you thinking-”

“Specific cases aren’t the point,” I said, too curtly. “The point is, for all we know I could be involved up to my tits already. And I don’t want to be looking over my shoulder for the rest of my life. I can’t live with that.”

He flinched. “It wouldn’t be for the rest of your life,” he said, quietly. “I hope I can promise you that much, at least. I do plan to get this fella, you know.”

I leaned back against the counter and took a breath. “I know, Sam,” I said. “I’m sorry. I didn’t mean it that way.”

“If, God forbid, he was after you, then all the more reason to stay out of the way and let me find him.”

The cheerful cooking smell had grown an acrid, dangerous edge: something was starting to burn. I switched off the cooker, shoved the pans to the back-neither of us was going to feel like eating for a while-and sat down cross-legged on the sofa, facing Sam.

“You’re treating me like your girlfriend, Sam,” I said. “I’m not your girlfriend, not when it comes to this kind of thing. I’m just another detective.”

He gave me a sad, sideways little smile. “Could you not be both?”

“I hope so,” I said. I wished I hadn’t finished the wine; this man needed a drink. “I really do. But not like this.”

After a while Sam let out a long breath, let his head fall back against the sofa. “So you want to do it,” he said. “Mackey’s plan.”

“No,” I said. “I just want to know about this girl. That’s why I said I’d go to the meeting. It’s got nothing to do with Frank and his wacko idea. I just want to hear about her.”

“Why?” Sam demanded. He sat up and caught both my hands, making me look at him. There was a ragged edge to his voice, something frustrated and almost pleading. “What’s she got to do with you? She’s no relation to you, no friend of yours, nothing. She’s happenstance, is all, Cassie: some girl who was looking for a new life and ran into the perfect chance.”

“I know,” I said. “I know, Sam. She doesn’t even sound like a particularly nice person; if we’d met, I probably wouldn’t have liked her. That’s the whole point. I don’t want her in my head. I don’t want to be wondering about her. I’m hoping that if I find out enough about her, I can drop the whole thing and forget she ever existed.”

“I’ve a double,” Sam said. “He lives in Wexford, he’s an engineer, and that’s all I know about the man. About once a year, someone comes up to me and tells me I’m the spit of him-half the time they actually call me Brendan. We have a laugh about it, sometimes they take a photo of me on their phones to show him, and that’s the end of that.”

I shook my head. “That’s different.”

“How?”

“For one thing, he hasn’t been murdered.”

“No harm to the man,” Sam said, “but I wouldn’t give a damn if he were. Unless I caught the case, it’d be no problem of mine.”

“This girl’s my problem,” I said. Sam’s hands were big and warm and solid around mine, and his hair was falling across his forehead like it always does when he’s worried. It was a spring Saturday night; we should have been walking on some beach down the country, surrounded by dark and waves and curlews, or making something experimental for dinner and playing music too loud, or settled in a corner of one of those rare out-of-the-way pubs where people still sing ballads when it gets past closing time. “I wish she wasn’t, but she is.”

“There’s something here,” Sam said, “that I’m not getting.” He had let our hands drop onto my knees and was frowning down at them, running his thumb around one of my knuckles in a steady, automatic rhythm. “All I’m seeing is a bog-standard murder case, with a coincidence that could happen to anyone. Sure, I got a shock when I saw her, but that’s only because I thought it was you. Once that was sorted, I figured everything would go back to normal. But you and Mackey, you’re both acting like this girl was something to you; like it’s personal. What am I missing?”