I had told Sam the truth: I never went to college. I came out of my A-levels, miraculously, with a B and two Ds-enough to have got me into some course somewhere, probably, except that I hadn't even filled in an application form. I told people I was taking a gap year, but the truth was that I wanted to do nothing, absolutely nothing, for as long as possible, maybe for the rest of my life.

Charlie was going up to London to study economics, so I went with him: there was nowhere else I particularly needed or wanted to be. His father was paying his share of the rent on a sparkly apartment with hardwood floors and a doorman, and there was no way I could afford my half, so I got a dingy little bedsit in a semi-dangerous area and Charlie got a flatmate, a Dutch exchange student who would be going home at Christmas. The plan was that by then I would have a job and be able to join him, but long before Christmas it became clear that I wouldn't be moving anywhere-not just because of money, but because I had, unexpectedly, fallen in love with my bedsit and my private, free-floating, wayward life.

After boarding school, the solitude was intoxicating. On my first night there I lay on my back on the sticky carpet for hours, in the murky orange pool of city glow coming through the window, smelling heady curry spices spiraling across the corridor and listening to two guys outside yelling at each other in Russian and someone practicing stormy flamboyant violin somewhere, and slowly realizing that there was not a single person in the world who could see me or ask me what I was doing or tell me to do anything else, and I felt as if at any moment the bedsit might detach itself from the building like a luminous soap bubble and drift off into the night, bobbing gently above the rooftops and the river and the stars.

I lived there for almost two years. Most of the time I was on the dole; occasionally, when they started hassling me or when I wanted money to impress a girl, I spent a few weeks working in furniture removals or construction. Charlie and I had, inevitably, drifted apart-starting, I think, with his look of polite, horrified fascination when he first saw the bedsit. We met for pints every couple of weeks, and sometimes I went to parties with him and his new friends (this is where I met most of the girls, including angsty Gemma with the drink problem). They were nice guys, his friends from uni, but they spoke a language I neither knew nor regretted, full of inside jokes and abbreviations and backslapping, and I found it hard to make myself pay attention.

I'm not sure what exactly I did for those two years. A lot of the time, I think, nothing. I know this is one of the unthinkable taboos of our society, but I had discovered in myself a talent for a wonderful, unrepentant laziness, the kind most people never know after childhood. I had a prism from an old chandelier hanging in my window, and I could spend entire afternoons lying on my bed and watching it flick tiny chips of rainbow around the room.

I read a lot. I always have, but in those two years I gorged myself on books with a voluptuous, almost erotic gluttony. I would go to the local library and take out as many as I could, and then lock myself in the bedsit and read solidly for a week. I went for old books, the older the better-Tolstoy, Poe, Jacobean tragedies, a dusty translation of Laclos-so that when I finally resurfaced, blinking and dazzled, it took me days to stop thinking in their cool, polished, crystalline rhythms.

I watched a lot of TV, too. In my second year there I became fascinated with late-night true-crime documentaries, mostly on the Discovery Channel: not with the crimes themselves, but with the intricate structures of their unraveling. I loved the taut, steady absorption with which these men-sharp FBI Bostonians, potbellied Texas sheriffs-carefully disentangled threads and joined jigsaw pieces, until at last everything fell into place and the answer rose at their command to hang in the air before them, shining and unassailable. They were like magicians, throwing a handful of scraps into a top hat and tapping it and whipping out-flourish of trumpets-a perfect, silken banner; only this was a thousand times better, because the answers were real and vital and there were (I thought) no illusions.

I knew it wasn't like that in real life, at least not all the time, but it struck me as a breathtaking thing to have a job where even that possibility existed. When, all in the same month, Charlie got engaged and the dole informed me they were cracking down on people like me and this guy with a thing for bad rap music moved in downstairs, it seemed like the obvious response to go back to Ireland, apply to Templemore Training College and start becoming a detective. I didn't miss the bedsit-I think I had been starting to get bored anyway-but I still remember those marvelous, self-indulgent two years among the happiest times of my life.

Sam left around 11:30; Ballsbridge is only a few minutes' walk from Sandymount. He gave me a quick, questioning look as he pulled on his coat. "Which way are you walking?"

"You've probably missed the last DART," Cassie told me easily. "You can crash on my sofa if you want."

I could have said I planned to take a taxi home, but I decided she had probably called it right: Sam wasn't Quigley, we wouldn't come in the next day to a gleeful little flurry of smirks and single entendres. "I think I have, actually," I said, checking my watch. "Would that be all right?"

If Sam was startled, he covered it well. "See ye in the morning, then," he said cheerfully. "Sleep tight."

"He fancies you," I told Cassie, when he had left.

"God, you're predictable," she said, digging in the wardrobe for the spare duvet and the T-shirt I keep there.

"'Oh, I want to hear what Cassie has to say, oh Cassie you're sooo good at this-'"

"Ryan, if God had wanted me to have a horrible pubescent brother, he would have given me one. Also your Galway accent sucks."

"Do you fancy him, too?"

"If I did, I would have done my famous trademark trick where I tie a cherry stem in a knot with my tongue."

"You do not either. Show me."

"I was joking. Go to bed."

We pulled out the futon; Cassie turned on the bedside lamp and I switched off the overhead light, leaving the room small and warm and shadowy. She found the knee-length T-shirt she sleeps in and took it into the bathroom to change. I tucked my socks into my shoes and pushed them out of the way under the sofa, stripped to my boxers, pulled on my T-shirt and settled myself under the spare duvet. We had the routine down pat by this time. I could hear her splashing water on her face and singing to herself, something folk-songy I didn't recognize, in a minor key. "To the Queen of Hearts is the Ace of Sorrow, he's here today, he's gone tomorrow…" She had pitched it too low; the bottom note disappeared into a hum.

"Do you really feel that way about our job?" I asked, when she came out of the bathroom (small bare feet, smooth calves muscled like a boy's). "The way Mark feels about archaeology?"

I had been saving the question till Sam left. Cassie gave me a quizzical little sideways grin. "I have never poured booze on the squad-room carpet. Cross my heart."

I waited. She slid into bed and leaned up on one elbow, her cheek on her fist; the glow of the bedside lamp edged her with light, so that she looked translucent, a girl in a stained-glass window. I wasn't sure she was going to answer, even without Sam there, but after a moment she said, "We're dealing with truth, finding truth. That's serious business."

I thought about this. "Is that why you don't like lying?" This is one of Cassie's quirks, especially odd in a detective. She omits things, eludes questions with open mischief or so subtly you hardly notice her doing it, spins misleading phrases with a conjurer's expertise; but I had never known her to lie outright, not even to a suspect.