"Oh for God's sake," I said, falling for it. Sam grinned and shook his head at me, and slid past me to a seat. I went back to looking at Cassie, who had sat down and propped one foot against the chair in front of her, leaning her notebook on her thigh.

She wasn't dressed like a Murder detective. You learn by osmosis, as soon as you set your sights on the job, that you are expected to look professional, educated, discreetly expensive with just a soupçon of originality. We give the taxpayers their money's worth of comforting cliché. We mostly shop at Brown Thomas, during the sales, and occasionally come into work wearing embarrassingly identical soupçons. Up until then, the wackiest our squad had got was this cretin called Quigley, who sounded like Daffy Duck with a Donegal accent and wore slogan T-shirts (MAD BASTARD) under his suits because he thought he was being daring. When he eventually realized that none of us were shocked, or even remotely interested, he got his mammy to come up for the day and take him shopping at BT.

That first day I put Cassie in the same category. She was wearing combat trousers and a wine-colored woollen sweater with sleeves that came down past her wrists, and clunky runners, and I put this down as affectation: Look, I'm too cool for your conventions. The spark of animosity this ignited increased my attraction to her. There is a side of me that is most intensely attracted to women who annoy me.

I didn't register her very much over the next couple of weeks, except in the general way that you do register any decent-looking woman when you're surrounded by men. She was being shown the ropes by Tom Costello, our resident grizzled veteran, and I was working on a homeless man found battered to death in an alleyway. Some of the depressing, inexorable flavor of his life had leaked over into his death, and it was one of those cases that are hopeless from the start-no leads, nobody saw anything, nobody heard anything, whoever killed him was probably so drunk or high he didn't even remember doing it-so my gung-ho newbie sparkle was starting to look a little patchy. I was also partnered with Quigley, which wasn't working out; his idea of humor was to reenact large segments of Wallace amp; Gromit and then do a Woody Woodpecker laugh to show you they were funny, and it was dawning on me that I'd been teamed up with him not because he would be friendly to the new boy but because nobody else wanted him. I didn't have the time or the energy to get to know Cassie. Sometimes I wonder how long we might have gone on like that. Even in a small squad, there are always people with whom you never get beyond nods and smiles in corridors, simply because your paths never happen to cross anywhere else.

We became friends because of her moped, a cream 1981 Vespa that somehow, in spite of its classic status, reminds me of a happy mutt with some border collie in its pedigree. I call it the Golf Cart to annoy Cassie; she calls my battered white Land Rover the Compensation Wagon, with the odd compassionate remark about my girlfriends, or the Ecomobile when she is feeling bolshie. The Golf Cart chose a viciously wet, windy day in September to break down outside work. I was on my way out of the car park and saw this little dripping girl in a red rain jacket, looking like Kenny out of South Park , standing beside this little dripping bike and yelling after a bus that had just drenched her. I pulled over and called out the window, "Could you use a hand?"

She looked at me and shouted back, "What makes you think that?" and then, taking me completely by surprise, started to laugh.

For about five minutes, as I tried to get the Vespa to start, I fell in love with her. The oversized raincoat made her look about eight, as though she should have had matching Wellies with ladybugs on them, and inside the red hood were huge brown eyes and rain-spiked lashes and a face like a kitten's. I wanted to dry her gently with a big fluffy towel, in front of a roaring fire. But then she said, "Here, let me-you have to know how to twist the thingy," and I raised an eyebrow and said, "The thingy? Honestly, girls."

I immediately regretted it-I have never been talented at banter, and you never know, she could have been some earnest droning feminist extremist who would lecture me in the rain about Amelia Earhart. But Cassie gave me a deliberate, sideways look, and then clasped her hands with a wet spat and said in a breathy Marilyn voice, "Ohhh, I've always dreamed of a knight in shining armor coming along and rescuing little me! Only in my dreams he was good-looking."

What I saw transformed with a click like a shaken kaleidoscope. I stopped falling in love with her and started to like her immensely. I looked at her hoodie jacket and said, "Oh my God, they're about to kill Kenny." Then I loaded the Golf Cart into the back of my Land Rover and drove her home.

She had a studio flat, which is what landlords call a bedsit where there is room to have a friend over, on the top floor of a semi-dilapidated Georgian house in Sandymount. The road was quiet; the wide sash window looked out over rooftops to Sandymount beach. There were wooden bookshelves crammed with old paperbacks, a low Victorian sofa upholstered in a virulent shade of turquoise, a big futon with a patchwork duvet, no ornaments or posters, a handful of shells and rocks and chestnuts on the windowsill.

I don't remember very many specifics about that evening, and according to Cassie neither does she. I can remember some of the things we talked about, a few piercingly clear images, but I could give you almost none of the actual words. This strikes me as odd and, in certain moods, as very magical, linking the evening to those fugue states that over the centuries have been blamed on fairies or witches or aliens, and from which no one returns unchanged. But those lost, liminal pockets of time are usually solitary; there is something about the idea of a shared one that makes me think of twins, reaching out slow blind hands in a gravity-free and wordless space.

I know I stayed for dinner-a studenty dinner, fresh pasta and sauce from a jar, hot whiskey in china mugs. I remember Cassie opening a huge wardrobe that took up most of one wall, to pull out a towel for me to dry my hair. Someone, presumably her, had slotted bookshelves inside the wardrobe. The shelves were set at odd, off-kilter heights and packed with a wild variety of objects: I didn't get a proper look, but there were chipped enamel saucepans, marbled notebooks, soft jewel-colored sweaters, tumbles of scribbled paper. It was like something in the background of one of those old illustrations of fairy-tale cottages.

I do remember finally asking, "So how did you end up in the squad?" We had been talking about how she was settling in, and I thought I had dropped it in pretty casually, but she gave me a tiny, mischievous smile, as though we were playing checkers and she had caught me trying to distract her from a clumsy move.

"Being a girl, you mean?"

"Actually, I meant being so young," I said, although of course I had been thinking of both.

"Costello called me 'son' yesterday," Cassie said. "'Fair play to you, son.' Then he got all flustered and stammery. I think he was afraid I'd sue."

"It was probably a compliment, in its own way," I said.

"That's how I took it. He's quite sweet, really." She tucked a cigarette in her mouth and held out her hand; I threw her my lighter.

"Someone told me you were undercover as a hooker and ran into one of the brass," I said, but Cassie just tossed the lighter back to me and grinned.

"Quigley, right? He told me you were an MI6 mole."

"What?" I said, outraged and falling straight into my own trap. "Quigley is a cretin."