That's me — there is absolutely nothing I can do about it. Believe me, I wish I could.

     'I… I don't know…' Box said, looking at the immobile man and then fixing his colleague with a 'don't-you-dare-say-anything-else' stare. Cox made sure her blonde ponytail was tight and continued to scour the room.

     A small middle-aged woman hurried in. She was wearing thick mascara and foundation, but Box noticed the partially masked yellow of bruising around her left eye.

     'Oh good god,' the woman gave a small scream, covering her mouth with her hand. She started weeping.

     Box put an awkward hand on her shoulder. 'I'm Detective Inspector Box,' he said. 'I'm in charge. You are?'

     'Sally Nichol… Is Daddy OK? What's happened to my husband?' she said between sobs. 'I came as quickly as I could, Margaret rang me after she called you. She told me to get here right away.'

     Box hated grown women who called their old man Daddy. 'Margaret?'

     'Daddy's nurse. She comes at 7.30am every day.'

     Box motioned to the man in the wheelchair. 'Your father, can he… hear and see us?'

     Yes I can.

     'He has locked-in syndrome. He can't move any part of his body, but his mind is perfectly normal.' She placed her arms around his neck and hugged him. 'He needs changing.'

     She went to push the wheelchair. 'You'll have to leave him there for a moment,' Box said, wincing at the awkwardness. He took Sally by the shoulder and steered her out of the room, into a red stone-tiled hallway that, he had noticed on the way through, had a bathroom, a study, a kitchen and a lounge leading from it — an entire ground floor wing of this place was bigger than most people's houses.

     The forensics team arrived in their overalls, setting up equipment. Box had no idea what the criminal or ethical procedures were for a situation like this. It was too early to think properly. Why couldn't he have been assigned a bog-standard murder? Some gang-on-gang killing. He stifled a yawn, ran a hand over his bald head, and looked at his Casio watch: 8.17am; he badly needed a cigarette, a coffee and a bacon sandwich.

     He stuck to what he knew: 'Where were you last night?'

     Sally looked at him as though he had punched her in the face.

     'We have to rule everybody out,' Box reassured her. 'Standard procedure.'

     'I was at home, by myself. Clive and I have been going through a difficult patch. We'd rowed and he had come over here. He liked playing trains where Daddy could watch. Until Margaret called I assumed he'd slept over like he often does. As you can see, there's plenty of room. There are seven bedrooms upstairs and it's just Daddy here.'

     'I'll need you to head down to the station and give a statement to that effect. We'll be taking one from the nurse as well. Sergeant Cox, if you would be so kind as to show Mrs Nichol to one of the officers who will give her a lift.'

     'What about Daddy?'

     'I'm afraid right now he is both a potential witness, and a piece of evidence.'

'So it's up for "yes" and down for "no"?' Box said.

     'Yes that's right,' the doctor replied. 'Please be patient. You must understand how stressful this is for him.'

     You have no idea.

     They were back in the room, congregated around the man's bed, which was now inclined so that he was resting at a forty-five degree angle. He stared straight ahead. His lower lip hung down, exposing his gum. A makeshift police-issue recording device had been placed on the bedside table. It had taken three weeks to arrange a special permit to conduct the interview here. The man could have been wheeled down to the station, but his doctor had insisted the ordeal would be too much. Cox hit the record button and a red light came on.

     Box was anxious to finally get to the bottom of this. Sally and Margaret's prints were found on the knife — amongst those of a few other housekeepers and nurses who were quickly ruled out — and though both displayed no obvious signs of guilt under preliminary questioning, neither had a sound alibi. The nurse was at home looking after her two toddlers.

     The case was frustrating Box because they had a witness who had sat right in front of the crime; he just couldn't speak. This, at last, would finally resolve it. The doctor wiped a gobbet of spit from the man's chin. She was in her late 30s, tall — a shade under Box's six-foot-two — and slender with dark hair down to the small of her back. Box was attracted to her, making the close proximity and the job at hand harder. He felt awkward around attractive women. He wished he did not have the white shirt on with a ketchup stain on the collar.

     Box leaned towards the man and spoke in a loud voice, as if he was explaining directions to a tourist. 'Hello — Mr — Vermont — can — you — understand — me?'

     Of course I can — why is it so hard for people to understand that?

     The man looked up.

     'For the sake of the tape, in front of three verified witnesses, his pupils moved upwards,' Cox said.

     'You don't need to patronise him,' the doctor said to Box, 'he can understand you completely.'

     Box blushed. He ran through a series of yes and no questions to ascertain for the tape — and ultimately a judge and jury — that the man was at the scene of the crime. Cox held up recent photographs of Sally and Margaret to confirm he recognised them.

     In hers Sally was holding a glass of champagne on a beach in the Maldives, wearing a floral cotton dress. Her makeup did not quite hide the lines on her face and she was a few pounds overweight.

     You are so beautiful, my lovely little girl.

     Box thought Sally looked remarkably similar to Margaret. In her photo the nurse was a younger, thinner version of her employer. She could've been her daughter.

     Oh how I despise you, you horrid woman.

     Finally Cox held up a photo of the victim, pot-bellied with a shaved head and podgy face.

     I certainly won't miss you.

     'Did you see who killed Clive?' Box said.

     The man looked up.

     'Was it Sally?'

     My precious Sally, she wouldn't hurt a fly.

     The man looked down.

     'Was it Margaret?'

     This feels every bit as good as I thought it would.

     The eyes flicked up in the frozen face.

     'Did she place the murder weapon in your hand?'

     If only I could tell you the times she waved that thing around in front of my face, holding the blade so close that it almost touched my eyeball, threatening to do all sorts to me. The way she revelled in the power.

     The man looked up again.

Within the hour, Margaret had been arrested in her cramped one-bed flat. The officers who picked her up were exhausted by the time they had climbed the sixteen flights of the high-rise council estate block, a twenty-minute walk from the Vermont house. They told Box that when they peered over her shoulder it did not look as though it had been cleaned for weeks. Her kids, they were informed, were with their granny down the corridor.

     Margaret's eyes were dark and bloodshot with tiredness as she sat opposite Box and Cox in the interview room at the station; her despair in sharp contrast to the pin-striped, crew-cut, clean-shaven solicitor next to her. She was shivering, slightly, from the chilly air conditioning coming from an old unit that made the place smell stale.

     'I didn't do it,' she said, quietly.

     'We have a witness who saw you stab him,' Box said. 'Admit it and we can all get out of here.' He let out a sigh, he had a half-built Lego model of the Death Star at home and was itching to get back to it. 'The net is closing, Margaret, there's no point fighting it, you'll only get more tangled.'

     'That man is protecting his daughter — the bitch!' The detectives flinched at her sudden outburst. 'It was her, I know it was. Clive hated her. You should've heard the things he'd say about her.'

     'We have your fingerprints on the murder weapon,' Cox said. 'We found traces of your hair and saliva on the body. We found your skin under his nails. Did he try and rape you? Had he been doing it for a while? We get that a lot with these cases where workers end up murdering their employer.'

     Box admired his partner's tone in these scenarios; she was challenging yet at the same time somehow warm, a composition he lacked. His method was more blunt force. His record of confessions was poor. Cox was half his age but she would probably be his boss within five years.

     Margaret let out a hacking laugh. 'Rape me? Rape? Clive and I were lovers. He was going to leave that little bitch Sally and take half of Daddy's nest egg. Why would I kill him?' Her dry lips curled into a snarl. 'I had nothing to gain from it.'

     The revelation took Box by surprise. If it was true, it didn't make sense for her to be the killer. Judging from the house, she had just lost out on a lot of money. Maybe the old boy — poor bugger — was protecting his daughter. He had a lot of money but he couldn't do much else for her.

     'All that woman ever did was treat me like shit,' Margaret continued. 'And she was just the same with Clive. That's why he wanted to leave her. She couldn't stand that I was younger and prettier than her. She couldn't stand that I had two little kids and she didn't have any.'

     Box and Cox continued to probe her for an hour. She did not stop insisting her innocence and eventually they took a break. A desk officer brought Margaret a Diet Coke. Cox had a jasmine tea — police officers had changed from his younger days, Box thought, as he took his strong black coffee outside so he could have a cigarette and mull it over. A local journalist had got wind of the case, Box had no idea how, and it was gaining some national interest. They had to get a conviction.

     Leaning against the pocked-marked wall of the station, he couldn't say with 100 per cent certainty it was the nurse. All the pieces were in place, but something was nagging at him. The witness did not look like he was all there but the doctor was certain he was completely with it. Granted, he might not implicate his daughter, but why would he pin it on the nurse? These rich families were always messed up. Box dropped the burning butt into his plastic coffee cup where it hissed out in the dregs. He would make one more visit to Sally before making a decision.

The tyres of the navy 1995 VW Golf crunched on the gravel as he drove up the long driveway to Sally's cottage the next morning. Cox sat uncomfortably in the passenger seat, trying to make sure the fabric of her expensive-looking black suit touched as little of the upholstery as possible. He had explained, on several occasions, that seat was reserved for Bruno his Labrador.

     Box pulled up next to a Range Rover and a Porsche in front of a thatched, half-timbered building. As they knocked and waited on the steps between two neatly-trimmed bushes, Cox brushed away a few stray dog hairs.

     Sally was wearing her hair tied up in bun and holding a duster. A radio was blaring out some power ballad Box didn't recognise. There were piles of boxes behind her in the large entrance lobby. She looked fresh, Box thought, as if a layer of stress had peeled from her face.

     'I'm clearing out Clive's things,' she said, jovially. 'It's got to be done some time. Please, come in. Daddy's here, too.'

     She muddled around in the kitchen and carried a cafetière of steaming coffee into the cream-carpeted living room that, Box calculated, was the size of the entirety of his pokey two-bedroom apartment. The smell of fresh baking wafted in and out of the room with her. Photographs of Sally and her father were all around the room; on the mantle piece above the soot-stained open fireplace and hanging in lines along the blue walls: at her graduation, posing on mopeds on an exotic holiday, swimming in a clear Mediterranean sea. Following his stroke they were in similar locations but now he was by her side in a wheelchair instead, fixed in identical poses. Box noticed there were no images of Clive anywhere.

     'Here he is,' Sally said, returning and guiding the wheelchair into the room.

     'Hello — Mr — Vermont,' Box said, too loudly. Cox, sunk into a large grey L-shaped sofa next to her parter, winced.

     Oh, you again.

     Sally perched on the edge of an antique velvet chair next to the wheelchair, holding his hand while his eyes shot up and down.

     'Has she been charged yet?' Sally said, in a sweet tone.

     'Not yet, Box said. 'Apologies for the nature of this visit. There is a matter of sensitivity we'd like to clear up.' He took a gulp of coffee, deliberately delaying.

     'Margaret claims to have been in a relationship with your husband,' Cox said, cutting in. It irritated him slightly how easily she carved through difficult situations.

     Sally's expression tightened. 'That is complete and utter nonsense. Clive and I were married for twenty-one years and, though we did not always agree on everything, he was not a cheat. Margaret is a lying whore.' She looked as if she might cry, but regained composure.

     If only you knew, my dear. They were at it for years. Right in front of my eyes; holding hands and kissing whenever it was just them around without you. They thought you'd never find out — it was not exactly like I could tell you. The times I spent willing you to ask me that one question — 'is my husband a cheating scumbag who is having it off with the nurse?' — so I could reply 'yes' almost killed me.

     'Quite,' Box said. 'She alleges that you killed your husband to stop him running off with your money.'

     Please let's not start this — I told you what happened.

     'Do I need a lawyer here?' Sally challenged him.

     'No — please — we just have some loose ends to tie up, that's all.' Another lawyer complicating matters was the last thing he wanted. They only had twelve more hours before they would have to charge or release Margaret.

     'She was always jealous of me,' Sally said. 'I had everything I ever wanted and she had very little. We paid her well, of course, above the going rate for a private nurse, but she was an envious cow. She wanted what I had.'

     Stay strong, this will be over soon.

     They continued the questions for twenty minutes, but Box didn't get the sense that she was lying. The nurse was either not in a relationship with the husband, or Sally didn't know about it.

     They were back in the car, waiting for the automatic gate to clunk open and Box lit up a cigarette. 'What do you think?'

     'It was definitely the nurse,' she said. 'She's concocted some bullshit story to make it seem like she would lose out significantly if she killed him when she knew we were on to her. She wasn't shouting about their supposed relationship the first time we spoke to her.'

     'I think you're right,' Box said. 'He's either got too heavy handed with her — like he clearly did with his wife — or she's finally lost it one night at the way they treated her.' He took a deep, satisfying drag. He was looking forward to getting home and a few weeks of quiet nights on the sofa.

     Back at the station, they charged Margaret Helmstead with murder.

Sally sat on the medical bed, twirling the pink flower between her thumb and forefinger. She had picked it from the patch of Red Campions growing in the garden and it was wilting, almost dead.

     She switched off the flat-screen television mounted at the foot of the bed, vanishing the reporter who had been detailing the events of Margaret's conviction. The case had been dubbed 'The Silenced Witness'. It was on every front page and top story on the news channels. Margaret's defence counsel had advised her to plead guilty, the reporter explained. Her solicitor, Jonathan Dembleby-Jones, told her that the evidence was insurmountable. The judge took her early plea and clean history into consideration and sentenced her to a minimum of fifteen years in prison.

     'It's done,' Sally said.

     I would cry, if I could, I'm so happy.

     The man's pupils bounced up and down between his eyelids. She held his hand and started to cry.

     'It's really over,' she said. 'For good.'

     There, there.

     She smoothed the silver wisps of hair that sparsely covered his scalp with her free hand.

     No-one will ever get away with hurting my baby.

     She composed herself, dabbing at the tears with a tissue and blowing her nose.

     'I'll just go and make a cup of tea and then we can play with the trains — that'll be nice, won't it?' She wheeled him over to the complex train set and absentmindedly placed the pink flower across the track nearest to the edge of the table, then left the room.

     The man's eyes bumped up and down in his sockets. The room was still and silent. Slowly, an arm reached out from the wheelchair. The fingers uncurled and clutched the train set's large remote control panel in a firm, strong grip, lifting it from the table.

     A black Holden B12 replica steam train sparked into motion. It hurtled around the track, through a tunnel running beneath a hill, past a station with tiny passengers always waiting. It crossed another set of tracks and gathered pace as it rounded a wide bend and came curving along the front of the table. The man's eyes followed its path as it beheaded the pink flower, crushing the petals into the track.