Sam Hollands was waiting for me outside.

‘Morning,’ she said, as I slid into the passenger seat. ‘Nice day for a mystery tour. Where did you say we’re going?’

‘St Albans.’

We drove off towards the main road.

‘I’m really grateful for this. I know you’ve probably got better things to do on your day off, Sam.’

‘Tell me again. You got a letter?’

It had been waiting for me when I got home from the shops yesterday. Nothing at all to indicate the nasty surprise that it contained – an ordinary envelope, my name and address typed on the outside, a first-class stamp, a smudged postmark. I read it out to Sam.

‘Is that it?’

‘What?’

‘It just seems like an abrupt way of ending a letter. You know, people who start a letter with “Dear Whoever” usually end it with “love from Whoever”, don’t they?’

We were on the M1, heading towards the M25. The traffic on the other side of the motorway flashed past us. I bit my lip.

‘Cathy…?’

‘There are a few more sentences on another page. It’s personal stuff.’

‘What sort of personal stuff?’

‘It’s nothing that will make any difference. Really.’

‘Cathy. This isn’t just a letter, it’s evidence. You know that, don’t you?’

‘Let’s wait and see what this is all about, shall we? It might be something really silly.’

‘What does Stuart make of it all?’

‘He’s away for a couple of days. Gone to a big new hospital in Belgium for a conference.’

She kept her eyes straight ahead and expressed her disapproval through the firm line of her mouth. I would end up showing her the letter anyway; I’d have to. But just for now I wanted to keep it between me and him.

‘What do you think it is?’ Sam asked.

‘I don’t know. I don’t think it’s anything good, put it like that.’

‘Me neither. I’m glad you rang me.’

‘I wondered if it was a trap.’

‘Well, he’s still safely inside, so you don’t need to worry about him being there to meet us. I rang the prison this morning.’

‘It’s not a prison letter,’ I said.

‘I noticed. He must have got someone to smuggle it out for him. Whatever happens, I’ll be putting in an intelligence report about that.’

We turned off the motorway and listened to Sam’s satellite navigation telling us in a calm voice to take the next turning left, right, continue straight for two point four miles.

‘So how’s Stuart?’

‘He’s fine. We’re fine.’

‘What’s it like being married?’

I laughed. ‘Not much different from how it was before. Anyway, it’s only been five months, give us a chance.’

‘No babies yet?’

‘Not yet. Don’t tell me you’re feeling broody?’

‘I’m not, but Jo is. We’re going to get married next year, I think.’

‘Sam, you never said.’

‘Well, we’ve been together ten years. It’s about time.’

‘Have you asked her?’

‘Not yet.’

‘You should get on and do it. It’s worth it. Can we come to the wedding?’

‘Of course you can. I was going to ask Sylvia, too.’

‘She’d love it.’

‘Anyway, we’re here.’

The Farley Industrial Estate was deserted, long wide streets empty of traffic, litter blowing across the pot-holed roads. We passed a kebab van, shutters down. Half the units were unoccupied, the whole area had a sense of desolation, and Unit 23 was no exception. It was as far as you could go, round a final corner. It was like the end of the world.

Sam parked the car in front of it.

‘Round there, look.’

Amidst the weeds growing around the building, a narrow dusty path twisted off between the chainlink fence and the wall of the unit. Stinging nettles grew to chest height, swaying towards us in the breeze.

Sam went first, weaving her way along the path, one hand on the wall of the unit. A rabbit scuttled across the path in front of us and made me jump.

Behind the unit the narrow space suddenly widened into a patch of wasteland. We walked across a large expanse of concrete, weeds growing up through the cracks. The sun shone over our heads and a bird sang from somewhere high up. It was completely deserted, not a person anywhere in sight.

‘Now where?’

I shaded my eyes from the sun and looked around, towards the trees he’d described, and saw it, a flash of colour in a landscape of grey and brown and green.

‘There. See it?’

It was a patch of red, scarlet, like a flag, and as we got closer it fluttered at us as though it were alive. I already knew what it was but it was still a shock to see it. I felt the tears start in my eyes and they were falling before I could stop them. It was like seeing an old friend, and a nightmare.

‘What is it?’ Sam said.

‘It’s my dress.’

The edges of it were ragged, and it was dusty and filthy, but I still recognised it. All of the buttons had gone from it, and sections of it had been cut out, leaving the bare edges to fray and catch the wind. It must have been here for some time.

‘That’s it? Just an old dress?’

It was anchored to the rocky soil by an old spade, rusted, which had been placed across it, and a heap of stones which had been laid over the top, like a cairn, like a grave.

‘No,’ I said. ‘It’s a marker.’

She saw it just a few moments after I did. At the bottom of the ditch, the movement caught my eye as the wind blew against a hank of dark hair. At first it looked artificial, like frayed hessian, and the skin like old canvas. And then the sudden whiteness of the broken bone, and there was no confusion any more.

‘Oh, shit, shit.’ Sam grabbed her mobile and started phoning, calling for back-up, and I sank to my knees amidst the dry soil and the stones, and stroked my fingers against the softness of the fabric for comfort.

‘I think she’s called Naomi,’ I said.

From the back pocket of my jeans I pulled out the second page of the letter.

‘Sam. You’d better look at this.’