Mackey pretended to think that over. ‘I like it. Not a lot, but I like it. She’s got no problem with the idea of us actually finding the witness and getting the goods, no? Landing in jail would just be a minor annoyance?’

‘She doesn’t think she’ll land in jail. That means she knows the card girl won’t rat her out. Either she knows it’s one of her own, and Joanne Heffernan’s bunch got mixed in along the way – by accident, or because Holly figured she might as well find out if they had any info while she was at it, since they were getting out at night as well, or because she just liked the idea of giving them a scare. Or else she’s got some hold over Heffernan’s lot.’

Mackey’s eyebrow was up. ‘I said she’s not thick, kid. I didn’t say she was Professor fucking Moriarty.’

I said, ‘Tell me that doesn’t sound like something you would do.’

‘I might well. I’m a pro. I’m not a naïve teenage kid whose entire experience of criminal behaviour is one unfortunate encounter seven years back. I’m flattered that you think I’ve raised some kind of evil genius, but you might want to save a little of that imagination for your online warcrafting time.’

I said, ‘So is Holly a pro. So are all of them. If I’ve learned one thing today, it’s that teenage girls make Moriarty look like a babe in the woods.’

Mackey gave me that with a tilt of his chin. Thought. ‘So,’ he said. ‘In this pretty little story, Holly knows the card girl won’t dob her in, but she’s still willing to take major risks to find out who it is. Why?’

‘If that was you,’ I said. ‘Starting to think about leaving school. Starting to realise that you and your friends are going to be heading out into the big wide world; this, what you’ve got now, it’s not going to last forever, you’re not always going to be bestest mates who’d die sooner than dob each other in. Would you want to leave a witness out there?’

I expected a punch, maybe. Got a startled snort of laughter that even sounded real. ‘Jesus, kid! Now she’s a serial killer? You want to check her alibi on the OJ case, too?’

I didn’t know how to say it, what I’d seen in Holly. Things turning solid, the world widening in front of her eyes. Dreams shifting to real, and the other way round, like a drawing sliding from charcoal to oil in front of your eyes. Words changing shape, meanings slipping.

I said, ‘Not a serial killer. Just someone who didn’t realise what she was starting.’