Frank’s sister Jackie was a hairdresser, so on Friday evening he brought her over to the flat, to cut my hair. Jackie was skinny, bleached blonde and totally unimpressed by her big brother. I liked her.

“Ah, yeah, you could do with a trim all right,” she told me, giving my fringe a professional riffle with long purple nails. “How do you want it?”

“Here,” Frank said, fishing out a crime-scene shot and passing it to her. “Can you do it to match this?”

Jackie held the photo between thumb and fingertip and gave it a suspicious look. “Here,” she said. “Is your woman dead?”

“That’s confidential,” Frank said.

“Confidential, me arse. Is that your sister, love?”

“Don’t look at me,” I said. “This is Frank’s gig. I’m only getting dragged along for the ride.”

“You wouldn’t want to mind him. Here-” She took another look at the photo and held it out to Frank at arm’s length. “That’s bleeding disgusting, so it is. Would you not think of doing something decent, Francis? Sorting out the traffic, something useful like that. Took me two hours to get into town from-”

“Would you ever just cut, Jackie?” Frank demanded, raking his hair exasperatedly so that it stood up in tufts. “And stop wrecking my head?” Jackie’s eyes slid sideways to mine and we shared a tiny, mischievous, female grin.

“And remember,” Frank said belligerently, noticing, “keep your mouth shut about this. Clear? It’s crucial.”

“Ah, yeah,” Jackie said, pulling a comb and scissors out of her bag. “Crucial. Go and make us a cup of tea, will you? That’s if you don’t mind, love,” she added, to me.

Frank shook his head and stamped off to the sink. Jackie combed my hair down over my eyes and winked at me.

When she was finished I looked different. I had never had my fringe cropped that short before; it was a subtle thing, but it made my face younger and barer, gave it the big-eyed, deceptive innocence of a model’s. The longer I stared in the bathroom mirror, that night while I got ready for bed, the less it looked like me. When I hit the point where I couldn’t remember what I had looked like to begin with, I gave up, gave the mirror the finger and went to bed.

On Saturday afternoon Frank said, “I think we’re just about good to go.”

I was lying back on the sofa with my knees hooked over the arm, going through the photos of Lexie’s tutorial groups one last time and trying to look blasé about this whole thing. Frank was pacing: the closer you get to the start of an operation, the less he sits down.

“Tomorrow,” I said. The word burned in my mouth, a wild clean burn like snow, taking my breath away.

“Tomorrow afternoon-we’ll start you off with a half day, ease you into it. I’ll let the housemates know this evening, make sure they’re all there to give you a nice warm welcome. Think you’re ready?”

I couldn’t imagine what, on an operation like this, could possibly constitute “ready.” “Ready as I’ll ever be,” I said.

“Let’s hear it once more: what’s your goal for Week One?”

“Not to get caught, mainly,” I said. “And not to get killed.”

“Not mainly; only.” Frank snapped his fingers in front of my eyes on his way past. “Hey. Concentrate. This is important.”

I put the photos down on my stomach. “I’m concentrating. What?”

“If someone’s going to suss you, it’ll be in the first few days, while you’re still finding your feet and everyone’s looking at you. So for Week One, all you do is ease your way in. This is hard work, it’ll be tiring at first, and if you overdo it you’ll start slipping-and all it takes is one slip. So go easy. Take time out if you can: go to bed early, read a book while the others play cards. If you make it to next weekend, you’ll be into the swing of things, everyone else will have got used to having you back, they’ll barely be looking at you any more, and you’ll have a lot more leeway. Until then, though, you keep your head down: no risks, no sleuthing, nothing that could raise a single eyebrow. Don’t even think about the case. I don’t care if this time next week you don’t have one single piece of useful info for me, as long as you’re still in that house. If you are, we’ll reassess and take it from there.”

“But you don’t really think I will be,” I said. “Do you?”

Frank stopped pacing and gave me a long steady look. “Would I send you in there,” he asked, “if I didn’t think it could be done?”

“Sure you would,” I said. “As long as you thought the results would be interesting either way, you wouldn’t think twice.”

He leaned back against the window frame, apparently considering that; the light was behind him and I couldn’t see his expression. “Possible,” he said, “but irrelevant. Yeah, sure, it’s dicey as all hell. You’ve known that since Day One. But it can be done, as long as you’re careful, you don’t get spooked and you don’t get impatient. Remember what I said last time, about asking questions?”

“Yep,” I said. “Play innocent and ask as many of them as you can get away with.”

“This time is different. You need to do the opposite: don’t ask anything unless you’re absolutely sure you’re not meant to know the answer already. Which means, basically, don’t ask anyone anything at all.”

“So what am I supposed to do, if I can’t ask questions?” I had been wondering about this.

Frank crossed the room fast, shoved paper off the coffee table and sat down, leaning in to me, blue eyes intent. “You keep your eyes and ears wide open. The main problem with this investigation is that we don’t have a suspect. Your job is to identify one. Remember, nothing you get will be admissible anyway, since you can’t exactly caution the suspects, so we’re not gunning for a confession or anything like that. Leave that part to me and our Sammy. We’ll make the case, if you just point us in the right direction. Find out if there’s someone out there who’s managed to stay off our radar-either someone left over from this girl’s past, or someone she took up with more recently and kept a secret. If anyone who isn’t on the KA list approaches you-by phone, in person, whatever-you play them along, find out what they’re after and what the relationship was, and get a phone number and full name if you can.”

“Right,” I said. “Your mystery man.” It sounded plausible enough, but then Frank always does. I was still pretty sure that Sam was right and his main reason for doing this wasn’t because he thought it had a snowball’s chance in hell but because it was such a dazzling, reckless, ridiculous once-off. I decided I didn’t care.

“Exactly. To go with our mystery girl. Meanwhile, keep an eye on the housemates and keep them talking. I don’t rate them as suspects-I know your Sammy has a bee in his bonnet about them, but I’m with you, they don’t add up-but I’m pretty sure there’s something they’re not telling us. You’ll see what I mean when you meet them. It might be something completely irrelevant, maybe they just cheat on their exams or make moonshine in the back garden or know who’s the daddy, but I’d like to decide for myself what’s relevant here and what’s not. They’re never going to talk to cops, but if you go at it right, there’s a good chance they’ll talk to you. Don’t worry too much about her other KAs-we’ve got nothing that points to any of them, and Sammy and I will be on them anyway-but if anyone’s acting even slightly dodgy, obviously, report back to me. Got it?”

“Got it,” I said.

“One last thing,” said Frank. He unfolded himself from the table, found our coffee mugs and took them over to the kitchen. We had got to the point where there was always, every hour of the day or night, a large pot of strong coffee keeping warm on the cooker; another week and we would probably have been eating the grounds straight from the bag with a spoon. “I’ve been meaning to have a little chat with you for a while now.”