There were posters covering almost every inch of the pale green Anaglypta: the Spurs team of 1975, with Steve Perryman in front holding the ball; a futuristic Roger Dean landscape; the female tennis player walking away from camera scratching at a bare buttock. In the corner of the room, a music centre sat on a shelf supported by house-bricks, Bowie and Deep Purple gatefolds spread out across its Perspex lid and leaning against its speakers. Books and piles of magazines were strewn across an old dining table, carried up from downstairs to be used as a desk: Melody Maker, New Musical Express, Shoot!, Jaws, Chariots of the Gods, a couple of tattered Sven Hassel paperbacks. A Jilly Johnson calendar and a Woolworth’s dartboard on the wall next to the window…

Thorne blinked and looked again at these newer walls. Smooth and orchid pale.

There were reproductions of ancient maps, architectural blueprints with elaborate French calligraphy, posters for exhibitions at the V & A and Tate Modern. Some had been mounted in simple clip-frames while others were stuck to the wall with Blu-Tack. Standing in the centre of a very different bedroom from the one that had once been his own, Thorne decided that what Parsons had said the day before was about right: Luke Mullen was hardly a typical sixteen-year-old.

He walked across to the metal and glass workstation, surprised to see an Arsenal diary on top of the papers stacked to one side. He reached for it, curious, and somewhat relieved that the boy – though clearly misguided in his choice of team – had at least one passion with which Thorne was able to identify. He flicked through the first few pages, saw immediately that it was no more than a homework diary.

There was a rectangular patch of dust on the glass, where Luke’s laptop had sat. The tech boys were still working on the hard drive, digging around for anything that might have been well hidden by anyone who knew what they were doing. But from what they’d been able to establish thus far, there was no significant email correspondence, nothing on any computerised diary to suggest that Luke had been planning to go anywhere. He hadn’t spent time in chat rooms, and it didn’t appear that he’d struck up a recent relationship with anyone online.

Little more had been gleaned from the details of his mobile-phone activity. The phone itself had been in Luke’s possession when he’d gone missing, so it had not been possible to check his contacts list, but records of calls and text messages provided by the phone company had yet to reveal anything that looked important. Luke had called his sister more than anybody else.

Thorne stared at the dust, at the shape of it, marking the absence of something, and found himself holding his breath. He imagined a young, alert mind racing, and fighting hard as the drug took hold, as eyelids dropped and thoughts slipped into the wet. Sopping and inky-black…

He pulled down the sleeve of his jacket, gripped it between fingers and palm and leaned down to wipe away the marks from the glass.

‘You won’t find him in here.’

Thorne turned to see Juliet Mullen standing in the doorway of her brother’s room. He slapped the grey dust marks from his sleeve. ‘Actually, I’ve found quite a lot of him,’ he said.

The girl rolled her eyes and walked past him into the room, clearly unimpressed, and unwilling to discuss anything as tedious as an abstract concept. She leaned back against a wall and slid slowly down it until she was sitting on the grey carpet. ‘So…?’

Thorne looked around, then back at Juliet. ‘Well, Luke was certainly tidy.’

‘Nothing gets past you, does it?’

‘I am a detective.’

‘Can you prove that?’

‘I’ve taken exams.’

‘They must have lowered the pass rate.’

She wasn’t smiling, but Thorne sensed that behind the studied air of boredom and irritation, it was a struggle not to; that she was enjoying the banter. Her hair was long, the same charcoal as the make-up around her eyes and the hooded top she wore over baggy jeans. Skateboarder chic, Thorne thought it might be called. Or grunge, or something. He thought about asking her, then decided it wasn’t such a great idea.

‘What was on the video?’ she said suddenly.

It took Thorne a moment to work out what she was talking about; a moment before deciding he would not answer.

‘Mum and Dad watched it this morning, before they called Porter. Just the once, I think, but it was enough. Obviously they wouldn’t let me see it. And they didn’t want to talk about it afterwards, so…’

‘So?’

‘So… I thought it couldn’t hurt to ask.’

Thorne watched her draw her knees up, shrinking into the corner of the room. He couldn’t help but be reminded of the previous evening with Phil Hendricks. Now, as then, he could see the pain and the longing beneath the pose; the anguish, raw behind the flippant remark. It couldn’t hurt to tell her.

‘It was Luke. Just Luke on the tape.’

She nodded quickly, as though something she already knew had been confirmed. It was a mature gesture, self-possessed, but in the next instant a tremor in the soft flesh around her mouth turned her back into a child again. ‘What did he say? Did he say anything?’

‘Juliet, I can’t-’

‘They were crying after they’d watched it, the pair of them. They pretended they weren’t, which was a bit bloody pointless, if you ask me. I mean, I knew what it was, you know? I didn’t think they were watching porno at nine o’clock in the morning.’

‘They didn’t want you to get upset,’ Thorne said.

‘Right, that’s brilliant. So now all I can think about is what might have been on the tape. What whoever’s got Luke might have been doing to him. How much pain he might have been in.’

‘He’s doing OK. Honestly.’

‘Define “OK”.’

Thorne took a deep breath.

‘“OK” as in having a whale of a time?’ She began plucking at the pile of the carpet. ‘Or “OK” as in still breathing?’

It was as tough a question as had been thrown at Thorne in a long time. ‘Nobody’s hurting him.’

Her head dropped to her knees. When she heaved it up again fifteen or twenty seconds later, the eyeliner was beginning to run. ‘He’s got a year and a bit on me, but sometimes it’s like I’m the older sister.’ Her eyes roamed from one part of the room to another, like she was searching to prove her point. ‘I have to look after him in loads of ways. You know what I mean?’

Thorne stepped across and sat down on the edge of the bed. The duvet was dark blue and neatly squared away. He guessed that Luke had probably made the bed himself before leaving for school on Friday. ‘Yeah, I think I do,’ he said.

She sniffed. ‘Pain in the fucking arse…’

The silence that followed was probably more uncomfortable for the girl than it was for Thorne. It was less than half a minute before she pulled herself to her feet. ‘Right…’ Like she had a lot to be getting on with.

Thorne stood, too. He cocked his head towards the doorway, towards the rest of the house. ‘It’s good that you’re all so… close. At a time like this, you know?’

Juliet Mullen nodded, pushed her hair back behind her ears.

‘What did they argue about?’ Thorne walked back to the workstation and looked at the photograph pinned to a corkboard above it: Luke on his father’s shoulders, eyes wide behind orange swimming goggles; the pair of them grinning like idiots and the sun bouncing off the blue water around them. ‘Luke and your dad, last Friday morning.’

‘Stupid stuff about school.’

‘Work stuff?’

‘About Luke not making the rugby team or something. It wasn’t a big deal.’

‘Your dad seems to think it was.’

‘That’s just because of what’s happened. Because he’s feeling guilty. Because the last time he saw Luke, the two of them were shouting at each other.’ She took a pace towards the bed and leaned down to smooth out the duvet where Thorne had been sitting. ‘Luke was already feeling bad about it by the time we got to school. He told me he was going to say “sorry” when he got home, that it was all his fault for being cheeky or whatever.’