'I've not done nothing to him. I've not seen him for months.' He hung his head. 'She won't let me.'

'You've been seen, Darren. People reported your car.' Logan tried to peer out through the kitchen window, but all he could see was himself staring back, reflected in the glass.

'I…' Darren sniffed. 'I used to drive round there. See if I could get a glimpse of him, you know, out playing or something? But she wouldn't let him out, would she? Wouldn't let him be like the other kids.'

Logan flicked the light-switch off, plunging the kitchen into darkness. Without the light turning the window into a mirror he could see out into the back garden. The pair of policemen he'd dispatched to watch the back were there, shivering away in the cold drizzle. There was a shed in one corner.

Smiling he snapped the lights back on, making everyone squint.

'What?'

'Come on,' he said, grabbing Darren by the collar, 'let's go take a look in the shed.'

But Richard Erskine wasn't in there. Just a Flymo, a couple of trowels, a bag of fertilizer and a pair of secateurs.

'Arse.' They stood in the lounge, drinking piss-poor tea. The room was crowded with two soggy PCs, the WPC, Darren Caldwell and Logan. The man of the house sat on the sofa, looking more and more unhappy with every minute that passed.

'Where is he?' asked Logan again. 'You're going to have to tell us sooner or later. Might as well be now.'

Darren scowled at them. 'I haven't seen him. I've got no idea what you're talking about.' 'OK then,' said Logan, perching on the arm of the lime-green settee, 'where were you yesterday morning at ten a.m.?'

Darren sighed theatrically. 'I was at work!'

'And you can prove this, can you?'

A nasty grin burst into life on Darren's face. 'Fuckin' right I can. Here-' he snatched the phone off the low coffee table and thrust it at Logan, before dragging a copy of the Yellow Pages out from beneath a pile of Hello! magazines. 'Broadstane Garage,' he said, pulling the thick, yellow directory open and flicking through it with angry fingers. 'Call them. Go on: speak to Ewan. He's my boss. Ask him where I was. Go on.'

As he took the phone and the Yellow Pages, Logan had a nasty thought: what if Darren was telling the truth?

Broadstane Garage had a display ad: something cheesy with a smiling spanner and a happy nut and bolt. The advert said 'Open 24 Hours' so Logan dialled the number. The ringing tone sounded in his ear, over and over and over. He was just about to hang up when a gruff voice shouted: 'Broadstane Garage!' in his ear.

'Hello?' said Logan, when his hearing had returned. 'Is this Ewan?'

'Who's this?'

'This is Detective Sergeant Logan McRae of Grampian Police. Are you Darren Caldwell's employer?'

The voice on the other end of the phone became instantly suspicious. 'What if I am? What's he done?'

'Can you tell me where Mr Caldwell was between the hours of nine and eleven yesterday morning?'

Darren sat back on the settee smiling his smug smile and Logan got that sinking feeling again.

'Helping me rewire a Volvo. Why?'

'You're sure?'

There was a small pause and then: 'Course I'm bloody sure. I was there. If he was somewhere else I'd've bloody noticed. Now what's this all about?'

It took another five minutes to get rid of him.

Logan put the phone down and tried to hide the disappointment in his voice. 'It seems we owe you an apology, Mr Caldwell,'

'Fucking right you do!' Darren stood up and pointed at the front door. 'Now why don't you get off your lazy arses and go look for my son?'

He was good enough to slam the door behind them.

They trailed off through the drizzle to the rusty Vauxhall Logan had signed for. All this way for nothing. And now he had no good news to give DI Insch. He just had to hope the performance had gone well tonight. Perhaps the inspector would be in a good mood and not to take a bite out of his backside.

The PC behind the wheel turned the engine over, the car windows rapidly steaming up. He cranked up the blowers, but it made little difference. Instead he pulled off his clip-on tie and tried to wipe the worst of the fog away. It just moved the fuzzy moisture around.

With a sigh they settled back to wait for the small patches of clear glass to creep up the windscreen.

'You think his alibi's for real?' asked the WPC in the back.

Logan shrugged.

'The garage is open twenty-four hours: we'll check it out on the way back into town.' But Logan already knew the alibi would hold. Darren Caldwell couldn't have snatched his son while the five-year-old went to the shops for milk and chocolate biscuits.

But he'd been so sure!

Eventually the blowers made enough of a dent in the fog to see out. The PC clicked on the headlights and pulled away from the kerb. They made a three point turn in the cul-de-sac and went back the way they'd come. Logan watched Darren's house slide past the passenger window. He'd been so sure.

As they drove through Portlethen, heading for the dual carriageway back to Aberdeen, Logan saw the lights of the big DIY stores and supermarket twinkling up ahead. The supermarket would have alcohol. And right now Logan thought that going home with a bottle of wine was a very good idea. He asked the PC driving to make a short detour.

While the others waited in the car Logan slumped round the shelves, piling crisps and pickled onions into his basket. They'd gone out expecting to find the missing kid alive and well, returning to Force Headquarters as heroes. Instead they were going back empty handed with Logan looking like an idiot.

He threw a bottle of Shiraz in on top of the crisps, cursing as he realized he'd crushed half of them. Looking sheepish he sneaked back to the snack aisle and swapped the salt-and-vinegar-flavoured crumbs for a fresh packet.

Imagine Darren Caldwell living in that little house, not allowed to see his son, still driving around Torry trying to catch a glimpse of him. Poor sod. Logan had never had children. There had been a sticky moment when a girlfriend was two weeks late, but thankfully nothing ever came of it. He could only imagine what it must be like to have a son and be completely excluded from his life.

There were only two checkouts open, one manned by a girl with more spots than skin, the other by an old man with a gnarled face and shaky hands. Neither of them seemed capable of working at much beyond a slow crawl.

The woman in front of him in the queue had bought about every kind of ready-meal imaginable: curry and chips, pizza and chips, chicken chowmein and chips, burgers and chips, lasagne and chips…There wasn't a single piece of fruit or vegetable in her trolley, but there were six two-litre bottles of Diet Coke and a chocolate gateau. So that was all right.

Logan let his attention wander while the ancient man fumbled with the barcode scanner and the pre-packaged dinners. All the little shops – the shoe repair place, the photo-lab, the dry cleaners and the one selling grotesque glass clowns and porcelain figurines – were in darkness, the shutters down. Anyone having a last-minute, life-or-death need for an ornamental Scottie dog playing the bagpipes would just have to come back tomorrow.

He shuffled forward a step as the woman started packing her mound of microwave meals into plastic bags.

A children's television theme blared out from somewhere near the exit and Logan looked up to see an old woman hovering over one of the children's rides – a blue plastic railway engine rocking serenely back and forth making 'chuff-chuff noises. He watched the old woman smiling and bobbing in time with Thomas the Tank Engine until the theme tune ended and the railway engine ground to a halt. Granny opened her handbag, pulled out her purse and rummaged unsuccessfully inside for enough change to make the ride start up again. A sad-looking little girl emerged from Thomas's innards. She took Granny's hand and walked slowly out the door, all the time looking regretfully back at the engine's grinning face.