Saturday, 25th of November

The house on the terrace was quiet on a Saturday afternoon, all week it had been like Glasgow Central on Fair Friday, but everybody was out today. Colin Anderson had the whole house to himself. He was lying on the sofa, nursing a large Merlot and two sore feet after helping Brenda make an early start on the Christmas shopping. He was musing at the wine, as it swirled round the contours of the glass, admiring the patterns it left in the light of the wood-burning stove. His grandchild, Baby Moses, was asleep in his basket at Anderson's feet. Nesbit, the fat Staffie, was curled up on the sofa, ears tucked in so he didn't hear the rain battering against the windows. American Beauty played on the DVD, with the volume too low to hear.

It was almost perfect yet Anderson was not at peace. He was still digesting the news that his partner for twenty years had resigned. Costello was gone. No notice. No chat. No goodbyes. She had walked into ACC Mitchum's office unannounced, uninvited and slapped her letter of resignation on the desk right in front of him.

Just like that.

Twenty years they had worked together, fought, made up and fallen out again, shared laughs, heartache and a few broken bones. She had always had his back. He had always had hers. At times, their thinking was polar, opposite points of the compass, balancing each other into a relationship that while turbulent, was effective. Their track record proved that. Now she was gone. Brenda, his wife, had explained it simply. The events of the last few months had been too intense. Costello had found Archie Walker. Anderson had found Baby Moses.

Both of them had moved on and maybe George Haggerty had been the catalyst that finally separated them.

But then Brenda would say that. She had never really liked Costello.

He checked his phone. He was meeting the guys tomorrow for fish and chips, a long-standing arrangement. Costello had been invited. She had declined.

Anderson could accept that she had resigned in a fit of pique, saying she could do more about Haggerty without the restriction of the badge. She thought 'killing the bastard' would do her more good than any counselling.

And she had been furious when her request to form a task force to investigate the murders of Abigail and Malcolm Haggerty had been refused. The case had been transferred to Complaints and Internal Investigations, purely for clarity and transparency, but to him, and Costello, it felt they themselves were being scrutinized and judged. The first two people on the murder scene were members of the law enforcement community, and not just any members; a DI and Chief Procurator Fiscal. And as the fiscal's goddaughter was the victim's sister, the press were having a field day.

Haggerty was now talking to the media, playing on the 'Monkey House of Horror' crap. The case had rarely been out the papers for the last six weeks. Every day there was another tasty morsel revealed by the press. One thing they were all agreed on: the police weren't coming out of it well. George Haggerty was the obvious suspect and he was the one man who couldn't have done it. Even ACC Mitchum let slip that he too, had taken a very close look at that alibi. He had personally interviewed the two police officers who had caught Haggerty speeding in his white Volvo on the A9. One obvious suspect. Police Scotland were his alibi.

Yet, Costello had persisted that George Haggerty had killed his family.

He looked down at the bundle of pink skin in the Moses basket. His grandson, his link with Haggerty, the one reason they kept in touch. Anderson didn't like Haggerty, not the way his daughter Claire did. God, she had even drawn him a portrait of Baby Moses in pastel and had left it for him, signed and wrapped. Anderson wished she hadn't bothered. There was nothing he could define, nothing he could specify, just a very intense feeling of dislike. If he himself had one tiny piece of physical evidence against Haggerty, Anderson would have brought him in and every bone in his body would have told him that he had the right bloke. Every time, he was in Haggerty's company, Anderson could sense smirking guilt.

Anderson watched the Merlot, tipping it to the left and right. 'He has a watertight alibi,' he said out loud, 'and no motive at all.' He looked at his grandson, blowing bubbles in his basket. 'Well, none that we have found.' Moses ignored him but Nesbit cocked an ear. 'George Haggerty did not kill his wife Abigail or his son Malcolm. He couldn't have done it.'

To his mind the best way of getting Costello back was to prove her wrong and get DCI Mathieson and her team to prove that somebody else did kill Abigail and Malcolm. Then maybe Costello could get closure and move on. And then she might come back into the fold as it were. He could see how the lack of progress in the case might have frustrated his colleague. The killer had ghosted in and out the house, without leaving a trace. Or a trace was there because it had a right to be there. The Haggertys were not a social couple so the only 'other' DNA in the house was Abigail's sister, Valerie Abernethy, and she had stayed overnight only a few days before the killings. No fingerprints, no footprints but the blood spatter had left a clean zone where the killer had stood and that indicated they were slim, five feet ten or more. George was five seven.

It had also really annoyed Costello to learn that Dali Despande's proposal to pilot a new fast-track child protection service had been side-lined, again. Looking back, Anderson thought, maybe she hadn't been right since the Kissel case, that child being starved to death, neglected by a mother who didn't care, let down by a failing social work system. It had taken that little boy weeks to die. Costello had sat in the court and relived every minute of the harrowing abuse. Then Malcolm? Costello had in her head that Malcolm was a vulnerable child.

Then she had walked into that scene, a scene so awful, it was reported that the crime scene photographer on duty had been off work since with stress, unable to cope with what he had seen.

Still none of it was any of his business. He had to walk away and leave it to Mathieson and Bannon. He had his cold case rapes to work on. Mitchum had given him one more week before the file went back to the freezer.

ACC Mitchum had been very clear; Anderson's loyalty was to the force.

Not that there was any conflict of loyalty, Costello had not been in contact for twenty-one days.

The Monkey House Of Horror.

The tabloids hadn't been able to resist that.

Valerie Abernethy looked up at the familiar ivy-covered eaves, the two red chimneys, the big, stained-glass window all hidden from the road by the majestic monkey puzzle tree. Had it been a happy family home for her sister? The gutter press thought so. A happy family home that became a scene of slaughter.

Valerie took a deep breath, trying to calm the panic. They wanted her to walk round the room where her sister had breathed her last, shielding her son from the blade of a knife. She was aware of the investigative team hovering at the bottom of the gravel drive, pretending they were giving her a little moment to catch her private thoughts. She knew she was under scrutiny.

Well, they could stand there, out in the rain, a little longer. Valerie placed her hand on a petal of the stained-glass flower, a delicate stem with Mackintosh roses. The glass felt slightly warm to her touch, almost soft under her fingertips.

The front door was familiar and welcoming, painted claret to match the colours of the roses. The brass knocker that Malcolm used to polish managed to shine, even in this God-awful weather. The door was open. They wanted her to go in alone.

She had no idea when she was last here. Her memory had large gaps.

A lump caught her throat. This was too difficult. She tried lifting her foot to get her up the step, one stride and she'd be in the house. Nothing happened. Her leg was leaden, stuck to the red tiles. Valerie recognized that feeling, an old enemy returning.

She needed a vodka.

She closed her eyes and stepped up. She had to do this for Abigail. For Malcolm.

She was now stock-still, one foot up, one foot down and with her fingertips still resting on the glass window. There was movement behind her. Archie Walker was about to intervene and offer his assistance.

She needed to do this on her own.

Valerie turned her face up to the sky and took a deep breath. The raindrops spat at her with disgust, stinging the skin of her cheek. She didn't think it would be as hard as this.

Did she remember that night six weeks ago? Could she remember, vaguely, walking out the hospital? Standing in the light rain in Great Western Road, watching the traffic? She was probably looking for an off licence. Then there was a smell of perfume she could recall, something familiar she recognized from Abigail's house. Was that merely an association of ideas, her imagination filling in the blanks?

Another pause.

A rustle of impatience from the drive.

That would be the boss, a small fascist detective with hard flinty eyes. That cop was mistaken if she thought her pillar-box red lipstick distracted from the incipient Hitler moustache. Her junior officer, the big bearded bloke, kept a good four paces behind her. Like Prince Philip.

Fascist and Beardy, it was easier than remembering their names.

Valerie heard footfall behind her as the cops and Archie, here in his role as her godfather, not in his professional role as the chief fiscal, were walking up the gravel driveway. They only moved because it was too wet for them to hang around outside but it still felt like harassment.

Bugger them. She would do exactly what DI Costello had done on the day she had discovered the bodies. Valerie pulled away from the front door and walked briskly round the house to the back garden.

Now she turned to confront Fascist and Beardy, wishing then away. They were standing across the path, blocking her way out. Archie gave her an encouraging smile. The rainwater ran down his face, to be cast off as he nodded his head. They were getting soaked through. Even better, Fascist had a sour look on her face, her lippy was about to run.

Valerie took a deep breath and walked in, recognizing immediately the stink of the forensic cleaning team, a scent she knew well from her days as a fiscal. This no longer smelled like Abigail's house; these rooms were no longer infused with the aroma of roses, fresh coffee and George's aftershave. She walked through the pristine utility room, the kitchen – everything neatly tidied away – to the back of the hall where her boots touched carpet for the first time. This was where Costello had spotted the tiniest smear of blood on the wall, blood that somebody had attempted to clean.

Valerie wondered how easy that had been to wipe away; probably easier to erase it from the wall than to erase from the memory. Fascist crept up behind her, and coughed in irritation.

'Is there anything missing that you notice?' she asked in her snippy voice. 'We have a comprehensive list of the items that Mr Haggerty has removed and we have the crime scene photographs and …' That earned her an elbow in the ribs from Archie, now standing beside her. Nobody wanted to be reminded of that.

'Anything missing?' confirmed Valerie, thinking that her sister's smile was 'missing', the hugs from Malcolm were 'missing'. The house was a mausoleum.

'Anything?'

Valerie looked around, climbed the stairs to the half landing and Primavera, resplendent in coloured glass on the west- facing window. The view east was totally obliterated by the monkey puzzle tree. It was an easy escape route; this window, down to the roof of the porch, a short slither to the ground. It was reported Malcolm had tried to escape that way once after an argument with his father. This was actually an easy house to gain entry and exit without being observed; the monkey puzzle tree hid a lot. She turned to look down at her companions, then up through the balusters to the upper landing, with its expensive Persian rug on an expanse of oak flooring. And a plain magnolia wall. Valerie screwed her eyes up to concentrate on what she wasn't seeing.

'Well, there was a picture there, a pastel. I suppose George took that, he always liked it.'

'What was the picture? I don't think he has mentioned it.' Bannon checked his iPad.

'A painting, it was a painting. A rowing boat on a canal, under willows, weeping willows. How fitting is that?' She turned to the other three. 'Uncle Archie? Did you say there was music playing when you … found them?'

Archie nodded, teary. 'Yes, that kid's song, it was on repeat on the CD. It had been playing for hours. "The Clapping Song", the one w- where …'Archie stuttered. 'Where the monkey got choked and they all —'

Valerie stared at the gap on the wall. 'They all went to heaven in a little rowing boat.'

Kieran Cowan drove along the loch side, through the dark night and the streaming rain. The engine of Ludwig, his 1977 Volkswagen Camper, hummed along nicely as the windscreen wipers beat a regular tattoo on the glass. The left one squeaking at the end of its sweep, the right one responding a millisecond later with a resounding thunk. He had been intending to fix that, but after a fortnight of constant rain, he had got used to the noise. It provided an irregular backbeat to 'Life in the Fast Lane', which blasted out the old Clarion cassette player at full volume.

He was used to this road. He would be able to drive even if the wiper gave up the ghost and fell off completely, spinning over the top of the van and flying into the night sky. He had driven Ludwig to Ardnamurchan once with a cracked windscreen, sticking his head out the driver's window until he could pull over and punch the crazed glass out.

Cowan kept his eyes on the road, the narrow stretches where he had to slow, the wider stretches where he could put his foot down and the nasty bends where he needed to hug the rock wall in case he met a HGV over the white line.

The clock on the dash was saying it was half eight. He wasn't in a hurry per se; he was a little concerned about time. As long as it was dark.

The job needed to be done, sorted and over with.

He drove confidently now, one hand on the steering wheel and the other steadying the rucksack that rolled and yawed in the passenger seat. The camera had been borrowed from the university. He had signed it out on Friday night to be returned Monday morning. It was an expensive bit of kit, a Macro Scub 4 underwater video camera. It was fully charged and ready to go, safely tucked in the rucksack along with his flask of tomato soup and some sandwiches. He had no idea how long he was going to be here. As someone with a gift for stating the obvious once said, 'It took as long as it took.'

Cowan drummed his fingers on the steering wheel in time with 'Life In The Fast Lane' as he waited for a short procession of traffic to pass, and when the road was clear he put his foot down. Ludwig's air-cooled engine whirred in protest. He turned onto the road that hugged the North-West side of the loch and accelerated, cruising along, singing tunelessly with Glen or Don, as he checked the clock again. He was probably a little early. He could have stayed at his laptop and got a more of his essay done but he wanted to be there first and check out the lie of the land, get a good spot where he could stay hidden.

Covert breeds covert.

He pulled into the deserted car park of the Inveruglass visitor centre, putting his lights off first so as not to disturb anybody already there. The car park was not entirely empty, there was a Mini parked at the front, looking out over the water. Cowan gave it more than a passing glance, his heart thumping, in case this was who he was looking for. But the windows of the other car were steamed up. He judged it had been there for some time and it looked as though there was still somebody in it. Or it might be two heads in the driver's seat, a lovers' tryst, a quiet night out on the lochside.

But he was mindful there was somebody there and he wished that Ludwig did not have such a distinctive engine.

Tonight could be the night.