Sam made a furious, frustrated noise, shook his head and slumped back against the glass. “All I want,” he said, “is to find out whether this fella’s a killer, and arrest him if he is. I don’t give a damn what else he’s been doing in his spare time. He could have fenced the Mona Lisa and I wouldn’t care. If you seriously think he’s been passing antiques, we can hand him over to A and A once we’re done with him, but for now, he’s a murder suspect. Nothing else.”

Frank raised one eyebrow. “You’re assuming there’s no connection. Look at the pattern. Up until those five move in, Naylor’s brick-throwing and spray-painting his little heart out. Once they’re there, he takes one or two more shots and then, just like that”-he snapped his fingers-“all quiet on the western front. What, he thought those five were cute? He saw them renovating and didn’t want to mess up the new decor?”

“They went after him,” Sam said. The set of his mouth: he was inches from losing his temper. “He didn’t fancy getting the shite kicked out of him.”

Frank laughed. “You think that kind of grudge vanished overnight? Not a chance. Naylor found some other way to do damage to Whitethorn House-otherwise he wouldn’t have quit the vandalism, not in a million years. And look what happens as soon as Lexie’s not there to slip him antiques any more. He gives it a few weeks, in case she gets back in touch, and when she doesn’t, he’s right back to the rock through the window. He wasn’t worried about getting the shite kicked out of him the other night, was he?”

“You want to talk about patterns? Here’s a pattern for you. When the five of them chase him off, back in December, his grudge only gets worse. He’s not going to take on all of them at once, but he keeps spying on them, he finds out that one of them makes a habit of going out walking during his window of opportunity, he stalks her for a while and then he kills her. When he finds out he didn’t even get that right, the rage builds up again, till he loses control and bangs an arson threat through the window. How do you think he feels about what happened the other night? If one of those five keeps wandering around the lanes on her own, what do you think he’s going to do about it?”

Frank ignored that. “The question,” he told me, “is what we do with Little Johnny now. We can arrest him for burglary, vandalism, theft, whatever else we can come up with, and keep our fingers crossed that it loosens him up enough that he gives us something on the stabbing. Or we can stick this lot back under his bed, thank him kindly for helping us with our inquiries, send him home and see where he takes us.”

In a way, this fight had probably been inevitable all along, from the second Frank and Sam showed up at the same crime scene. Murder detectives are single-minded, focused on narrowing the investigation slowly and inexorably till everything extraneous is gone and the only thing left in their sights is the killer. Undercovers thrive on extraneous, on spreading their bets and keeping all their options open: you never know where tangents might lead, what unexpected game might poke its head out of the bushes if you watch every angle for long enough. They light all the fuses they can find, and wait to see what goes boom.

“And then what, Mackey?” Sam demanded. “Just supposing for a second that you’re right, Lexie was slipping the man antiques to sell, and Cassie gets their little operation going again. Then what?”

“Then,” Frank said, “I have a nice chat with A and A, I head down to Francis Street and buy Cassie a handful of lovely shiny widgets, and we take it from there.” He was smiling, but his eyes on Sam were narrow and watchful.

“For how long?”

“As long as it takes.”

A and A uses undercovers all the time, undercovers posing as buyers, as fences, as sellers with nudge-and-a-wink sources, gradually working their way towards the big shots. Their operations last for months; they last for years.

“I’m investigating a fucking murder here,” Sam said. “Remember that? And I can’t arrest anyone for that murder while the victim’s alive and well and messing about with silver sugar bowls.”

“So? Get him after the antiques sting winds up, one way or the other. Best-case scenario, we establish a motive and a link between him and the victim, and we get to use them as leverage towards a confession. Worst-case scenario, we waste a little time. It’s not like our statute of limitations is about to run out.”

There wasn’t a snowball’s chance in hell that Lexie had spent the past three months selling John Naylor the contents of Whitethorn House just for kicks and giggles. Once that pregnancy test came up positive, she would have sold whatever it took to get out, but up until then: no.

I could have said so; should have. But the thing was that Frank was right on this much: Naylor would do anything that would damage Whitethorn House. He was going insane like a caged cat with his own helplessness, taking on that house charged with centuries of power, with no weapons in his hands but rocks and spray cans. If someone came up to him with a handful of spoils from Whitethorn House, a few bright ideas about where to sell the stuff and a promise of more, there was a good chance-an incredible chance-that he wouldn’t know how to say no.

“I’ll make you a deal,” Frank said. “Have another go at Naylor-just you, this time; he and I aren’t really clicking. Take as long as you need. If he gives you something on the murder-anything at all, even a hint-we arrest him, we forget the whole antiques question, we pull Cassie in and we shut the investigation down. If he gives you nothing…”

“Then what?” Sam demanded.

Frank shrugged. “If your way doesn’t work, then you come back out here and we all have a little chat about my way.”

Sam looked at him for a long time. “No tricks,” he said.

“Tricks?”

“Coming in. Knocking on the door when I’m on the edge of getting something. That kind of thing.”

I saw a muscle flick in Frank’s jaw, but all he said was, blandly, “No tricks.”

“OK,” Sam said, on a deep breath. “I’ll give it my best shot. Can you hang on here for a bit?”

He was talking to me. “Sure,” I said.

“I might want to use you-bring you in, maybe. I’ll figure it out as I go.” His eyes went to Naylor, who had switched to singing “Follow Me Up to Carlow,” just loud enough to be distracting. “Wish me luck,” he said, straightening his tie, and he was gone.

“Did your boyfriend just insult my virtue?” Frank wanted to know, when the door of the observation room shut behind Sam.

“You can challenge him to a duel if you want,” I said.

“I play fair. You know that.”

“Don’t we all,” I said. “We’ve just got different ideas about what counts as fair. Sam isn’t sure yours matches up all that well with his.”

“So we won’t buy a time share in the Med together,” Frank said. “I’ll live. What do you think of my little theory?”

I was watching Naylor, through the glass, but I could feel Frank’s eyes raking the side of my head. “I don’t know yet,” I said. “I haven’t really seen enough of this guy to have an opinion.”

“But you’ve seen plenty of Lexie-secondhand, but still, you know as much about her as anyone does. Think she’d be capable of something like that?”

I shrugged. “Who knows? The whole thing about this girl is that no one has a clue what she was capable of.”

“You were playing your cards very close to the chest, just now. It’s not like you to keep your mouth shut for that long, not when you must have an opinion one way or the other. I’d like to have some idea which side you might be on, if your fella comes out of there with nothing and we have to pick up this argument again.”

The interview-room door opened and Sam came in, juggling two mugs of tea and catching the door with his shoulder. He looked wide awake, almost jaunty: the fatigue falls off you, the second you’re face to face with a suspect. “Shh,” I said. “I want to watch this.”