“In a way,” I said, “it is personal, yeah. For Frank, partly it’s exactly what you said-he thinks this would all be a big brilliant adventure. But it’s more than that. Lexie Madison started out as his responsibility, she was his responsibility for eight months while I was under, she’s his responsibility now.”

“But this girl isn’t Lexie Madison. She’s an identity thief; I could go down to Fraud in the morning and find you hundreds more just like her. There is no Lexie Madison. You and Mackey made her up.”

His hands had tightened on mine. “I know,” I said. “That’s sort of the point.”

The corner of Sam’s mouth twisted. “Like I said. The man’s mad.”

I didn’t exactly disagree with him. I’ve always thought one of the reasons for Frank’s legendary fearlessness is that, way deep down, he’s never quite managed to connect with reality. To him every operation is one of those war games the Pentagon plays, only even cooler, because the stakes are higher and the results are tangible and long-lasting. The fracture is small enough and he’s smart enough that it never shows in obvious ways; but, while he’s keeping every angle covered and every situation beautifully, icily under control, some part of him truly believes he’s being played by Sean Connery.

I spotted this because I recognize it. My own border fence between real and not-real has never been all that great. My friend Emma, who likes things to add up neatly, claims that this is because my parents died when I was too young to take it in: they were there one day and gone the next, crashing through that fence so hard and fast they left it splintered for good. When I was Lexie Madison for eight months she turned into a real person to me, a sister I lost or left behind on the way; a shadow somewhere inside me, like the shadows of vanishing twins that show up on people’s X-rays once in a blue moon. Even before she came back to find me I knew I owed her something, for being the one who lived.

This was presumably not what Sam wanted to hear; he had enough on his plate without adding several new flavors of weird into the mix. Instead-it was the closest I could get-I tried to tell him about undercover. I told him how your senses are never quite the same again, how colors turn fierce enough to brand you and the air tastes bright and jagged as that clear liqueur filled with tiny flakes of gold; how the way you walk changes, your balance turns fine and taut as a surfer’s, when you spend every second on the shifting edge of a fast risky wave. I told him how afterwards I never shared a spliff with my mates or took E in a club again, because no high could ever compare. I told him how damn good at it I was, a natural, better than I’ll be at DV in a million years.

When I finished, Sam was looking at me with a worried little furrow between his eyebrows. “What are you saying?” he asked. “Are you saying you want to transfer back into Undercover?”

He had taken his hands off mine. I looked at him, sitting across the sofa with his hair rucked up on one side, frowning at me. “No,” I said, “that’s not it,” and watched his face clear in relief. “That’s not it at all.”

This is the part I didn’t tell Sam: bad stuff happens to undercovers. A few of them get killed. Most lose friends, marriages, relationships. A couple turn feral, cross over to the other side so gradually that they never see it happening till it’s too late, and end up with discreet, complicated early-retirement plans. Some, and never the ones you’d think, lose their nerve-no warning, they just wake up one morning and all at once it hits them what they’re doing, and they freeze like tightrope walkers who’ve looked down. This guy McCall: he’d infiltrated an IRA splinter group and nobody thought he even knew what fear was, till one evening he phoned in from an alleyway outside a pub. He couldn’t go back in there, he said, and he couldn’t walk away because his legs wouldn’t stop shaking. He was crying. Come get me, he said; I want to go home. When I met him, he was working in Records. And some go the other way, the most lethal way of all: when the pressure gets to be too much, it’s not their nerve that breaks, it’s their fear. They lose the capacity to be afraid, even when they should be. These can’t ever go home again. They’re like those First World War airmen, the finest ones, shining in their recklessness and invincible, who got home and found that home had no place for what they were. Some people are undercovers all the way to the bone; the job has taken them whole.

I was never afraid of getting killed and I was never afraid of losing my nerve. My kind of courage holds up best under fire; it’s different dangers, more refined and insidious ones, that shake me. But the other things: I worried about those. Frank told me once-and I don’t know whether he’s right or not, and I didn’t tell Sam this either-that all the best undercovers have a dark thread woven into them, somewhere.

So, on Sunday evening, Sam and I went to Dublin Castle for Frank’s council of war. Dublin Castle is where the Murder squad works. I had cleared out my desk there on another long cool evening, in autumn: stacked my paperwork in neat piles and labeled each pile with a Post-it, thrown away the cartoons stuck to my computer and the chewed pens and old Christmas cards and stale M amp;Ms in my desk drawers, turned off the light and closed the door behind me.

Sam picked me up. He was very quiet. He had been up and out early that morning, so early that the flat was still dark when he leaned over to kiss me good-bye. I didn’t ask him about the case. If he had found anything good, even the slimmest lead, he would have told me.

“Don’t let your man pressure you,” he said, in the car. “Into doing anything you don’t want to.”

“Come on,” I said. “When have I ever let anyone pressure me into anything?”

Sam adjusted his rearview mirror, carefully. “Yeah,” he said. “I know.”

When he unlocked the door, the smell of the building came at me like a shout: an old, elusive smell, damp and smoke and lemon, nothing like the antiseptic tang of DV in the new building up in Phoenix Park. I hate nostalgia, it’s laziness with prettier accessories, but every step hit me straight in the gut with something: me running down those stairs with a bunch of files in each hand and an apple caught between my teeth, my partner and me high-fiving each other outside that door after getting our first confession in that interview room; the two of us double-teaming the superintendent down that hallway, one in each ear, trying to hassle him into giving us more overtime. It seemed like the corridors had an Escher look, the walls all tilting in subtle, seasick ways, but I couldn’t focus my eyes enough to figure out exactly how.

“How’re you doing?” Sam asked quietly.

“Starving,” I said. “Whose idea was it to do this at dinnertime?”

Sam smiled, relieved, and gave my hand a quick squeeze. “We don’t have an incident room yet,” he said. “Till we decide… well, how we’re doing this; where we’re working out of.” Then he opened the door of the Murder squad room.

Frank was straddling a backwards chair at the head of the room, in front of the big whiteboard, and all his reassurances about a casual chat between him and me and Sam had been bollocks. Cooper, the state pathologist, and O’Kelly, the superintendent of the Murder squad, were sitting at desks on opposite sides of the room with their arms folded, wearing identical narky looks. This should have been funny-Cooper looks like a heron and O’Kelly looks like a bulldog with a comb-over-but actually it gave me a very bad feeling. Cooper and O’Kelly hate each other; getting them in the same room for any length of time takes a lot of skilled persuasion and a couple of bottles of pretty serious wine. For some cryptic reason of his own, Frank had pulled out all the stops to get them both there. Sam shot me a wary, warning glance. He hadn’t been expecting this either.