Plenty of murder cases turn into knock-down-drag-out battles of wits, but this was different. This was the first time I had felt like my real opponent wasn’t the murderer but the victim: defiant, clenching her secrets white-knuckle tight, and evenly, perfectly matched against me in every way, too close to call.

By Saturday lunchtime I had made myself nuts enough that I climbed up on the kitchen counter, took down my Official Stuff shoebox from the top of a cupboard, dumped the documents on the floor and went through them for my birth cert. Maddox, Cassandra Jeanne, female, six pounds ten ounces. Type of birth: single.

“Idiot,” I said, out loud, and climbed back up on the counter.

That afternoon, Frank called round. At this stage I was so stir-crazy-my flat is small, I’d run out of stuff to clean-that I was actually glad to hear his voice over the intercom.

“What year is it?” I asked, when he reached the top of the stairs. “Who’s the president?”

“Quit bitching,” he said, giving me a one-armed hug around the neck. “You’ve got this whole lovely flat to play in. You could be a sniper stuck in a hide, not moving a muscle for days on end and pissing into a bottle. And I brought you supplies.”

He handed me a plastic bag. All the main food groups: chocolate biscuits, smokes, ground coffee and two bottles of wine. “You’re a gem, Frank,” I said. “You know me too well.” He did, too; four years on, and he had remembered I like Lucky Strike Lights. The feeling wasn’t a reassuring one, but then he hadn’t intended it to be.

Frank raised a noncommittal eyebrow. “Got a corkscrew?”

My antennae went up, but I can hold my booze fairly well, and Frank had to know I wasn’t stupid enough to get drunk with him. I threw him a corkscrew and rummaged for glasses.

“Nice place you’ve got here,” he said, going to work on the first bottle. “I was scared I’d find you in some foul yuppie apartment with chrome surfaces.”

“On a cop’s salary?” Dublin housing prices are a lot like New York ones, except that in New York, you get New York for your money. My flat is one mid-sized room, on the top floor of a tall converted Georgian house. It has the original wrought-iron fireplace, enough room for a futon and a sofa and all my books, a tipsy slant to the floor in one corner, a family of owls living in the roof space, and a view of Sandymount beach. I like it.

“On two cops’ salaries. Aren’t you going out with our boy Sammy?”

I sat on the futon and held out the glasses for him to pour. “Only for a couple of months. We’re not at the living-in-sin stage yet.”

“I thought it was longer. He seemed pretty protective on Thursday. Is it true love?”

“None of your business,” I said, clinking my glass against his. “Cheers. Now: what are you doing here?”

Frank looked injured. “I thought you could use the company. I got to feeling guilty about leaving you stuck here, all on your own…” I gave him a dirty look; he realized it wasn’t working and grinned. “You’re too smart for your own good, do you know that? I didn’t want you getting hungry, or bored, or desperate for a smoke, and heading out to the shop. The odds are a thousand to one against you being spotted by anyone who knows our girl, but why take chances?”

This was plausible enough, but Frank has always had a habit of tossing lures in a few directions at once to distract you from the hook in the middle. “I’ve still got no intention of doing this, Frankie,” I said.

“Fair enough,” Frank said, unperturbed. He took a big swig of his wine and settled himself more comfortably on the sofa. “I had a chat with the brass, by the way, and this is now officially a joint investigation: Murder and Undercover. But your boyfriend probably already told you that.”

He hadn’t. Sam had stayed at his own place the last couple of nights (“I’ll be up at six, sure, no reason you should be as well. Unless you need me to come over? Will you be OK on your own?”); I hadn’t seen him since the murder scene. “I’m sure everyone’s delighted,” I said. Joint investigations are a pain in the hole. They always end up getting spectacularly bogged down in endless, pointless testosterone competitions.

Frank shrugged. “They’ll survive. Want to hear what we’ve got on this girl so far?”

Of course I did. I wanted it the way an alcoholic must want booze: badly enough to shove aside the hard knowledge that this was a truly lousy idea. “You might as well tell me,” I said. “Since you’re here.”

“Beautiful,” Frank said, rummaging through the plastic bag for the cigarettes. “OK: she first shows up in February 2002, when she pulls Alexandra Madison’s birth cert and uses it to open a bank account. She uses the birth cert, an account statement and her face to pull your old records from UCD, and she uses those to get into Trinity, to do a PhD in English.”

“Organized,” I said.

“Oh yeah. Organized, creative and persuasive. She was a natural at this; I couldn’t have done it better myself. She never tried to sign on the dole, which was smart; just got herself a job in a café in town, worked there full-time for the summer, then started at Trinity come October. Her thesis title is-you’ll like this-‘Other Voices: Identity, Concealment and Truth.’ It’s about women who wrote under other identities.”

“Cute,” I said. “So she had a sense of humor.”

Frank gave me a quizzical look. “We don’t have to like her, babe,” he said, after a moment. “We just have to find out who killed her.”

“You do. I don’t. Got anything else?”

He flipped a smoke between his lips and found his lighter. “OK, so she’s in Trinity. She makes friends with four other English postgrads, hangs out almost exclusively with them. Last September, one of them inherits a house from his great-uncle, and they all move into it. Whitethorn House, it’s called. It’s outside Glenskehy, just over half a mile from where she was found. On Wednesday night, she goes for a walk and never comes home. The other four alibi each other.”

“Which you could have told me over the phone,” I said.

“Ah,” Frank said, rummaging in his jacket pocket, “but I couldn’t have shown you these. Here we go: the Fantastic Four. Her housemates.” He pulled out a handful of photos and spread them on the table.

One of them was a snapshot, taken on a winter day, thin gray sky and a sprinkle of snow on the ground: five people in front of a big Georgian house, heads tilted together and hair blown sideways in a swirl of wind. Lexie was in the middle, bundled in that same peacoat and laughing, and my mind did that wild lurch and swerve again: When was I…? Frank was watching me like a hunting dog. I put the photo down.

The other shots were stills pulled off some kind of home video-they had that look, blurry edges where people were moving-and printed out in the Murder squad room: the printer always leaves a streak across the top right corner. Four full-length shots, four blown-up head shots, all taken in the same room against the same ratty wallpaper striped with tiny flowers. There was a huge fir tree, no decorations, caught in the corner of two of the shots: just before Christmas.

“Daniel March,” Frank said, pointing. “Not Dan, not God forbid Danny: Daniel. He’s the one who inherited the house. Only child, orphaned, from an old Anglo-Irish family. Grandfather lost most of their money in dodgy deals in the fifties, but there’s enough left to give Danny Boy a small income. He’s on a scholarship, so he doesn’t have fees to pay. Doing a PhD on, I kid you not, the inanimate object as narrator in early medieval epic poetry.”

“No idiot, then,” I said. Daniel was a big guy, well over six foot and built to match, with glossy dark hair and a square jaw. He was sitting in a wingbacked chair, delicately lifting a glass bauble out of its box and glancing up at the camera. His clothes-white shirt, black trousers, soft gray sweater-looked expensive. In the close-up his eyes, behind steel-rimmed glasses, were gray and cool as stone.