"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."