Just from memory, I was able to cross out entire evenings I knew we had spent together. On the weekends there were whole days I eliminated with a bold stroke of black: the Saturday we had taken the train to Brighton, walked on the beach, eaten some awful fish and chips, bought a secondhand book of poetry and I’d fallen asleep on his shoulder on the journey back; the day we walked along the Regent’s Canal from Kentish Town all the way to the river. Those were two days when he hadn’t been having sex with Milena Livingstone.

Then I started on the emails. At work, Greg had written twenty or thirty a day, sometimes more. Based on each one, I wrote ‘O’ for office in the appropriate slot on the card. Some were in clusters. He had a habit of sending a flurry of messages as soon as he arrived at work, another just before one o’clock and another at around five, but others were dotted through the day. It didn’t take me much more than an hour to work my way through the emails, and when I was done, I stood back and surveyed the result. The chart was already satisfyingly shaded in, and there was still so much to do.

The next day I invited Gwen round. I said it was urgent but she was at work and didn’t reach me until almost six. When she arrived I hustled her through to the kitchen, boiled the kettle and made a pot of coffee.

‘Would you like a biscuit?’ I said. ‘Or a slice of ginger cake? I made both this afternoon. I’ve been busy.’

Gwen looked amused and a bit alarmed. ‘Some cake,’ she said. ‘A tiny slice.’

I poured the coffee and gave her the cake on a plate. I wasn’t hungry. I’d felt I needed to cook but not to eat.

‘So what’s up?’ said Gwen. ‘Did you summon me here to try the cake? It’s great, by the way.’

‘Good, have some more. No, it’s nothing to do with that. Drink your coffee and I’ll take you through.’

‘Take me through? What is this, a surprise party?’

‘Nothing like that,’ I said. ‘I’ve got something to show you. I think it’ll interest you.’

Gwen took a few quick gulps of her coffee and said she was ready. I steered her along the hall and into the living room.

‘There,’ I said. ‘What do you think of that?’

Gwen stared down at the four large pieces of card, now covered with marks and stickers, all different shapes and colours. ‘It looks lovely,’ she said. ‘What’s it meant to be?’

‘That’s Greg’s life in the month before he died,’ I said.

‘What do you mean?’

I explained to Gwen how the charts represented days and sections of days. I told her about the timed emails and my own memories and how I’d even found receipts from the sandwich bars where Greg had bought his lunch. All the receipts, whether for food or petrol or stationery, gave not just a date but an exact time, to the minute, when the purchase was made. ‘So all these stickers, the yellow circles and the green squares, they show moments when I know exactly where Greg was. It’s pretty amazing, isn’t it?’

‘Yes, but -’

‘A couple of times a week Greg drove to visit a client. But I pretended to be Greg’s assistant, rang up and said that for tax reasons I needed an exact time for when the meeting had taken place. People were very helpful. I’ve marked all those in blue. Even then I was left with the gap between him leaving the office and arriving at the client. But I found a website. If I type in the postcode of the office and the postcode of his client, it gives an exact driving distance and even an estimated journey time. I’ve marked those in red. Obviously, driving in London traffic during the day, it’s not an exact science, but even so it fits pretty well. It took me a day and a half – and look.’

‘What?’

‘What do you see?’

‘Lots of colours,’ said Gwen. ‘Lots of stickers.’

‘No,’ I said. ‘It’s what you don’t see. There’s barely a gap over four weeks when I don’t know where he was and what he was doing.’

‘Which means?’

‘Look at the chart, Gwen,’ I said. ‘It shows Greg working very hard, travelling, eating, buying stuff, going to the movies with me. But where’s the bit when he’s having an affair? Where’s the space for him even to meet the woman he died with?’

There was a long pause.

‘Ellie,’ she began, ‘for God’s sake -’

‘No,’ I said. ‘Stop. Listen for a moment. I talked to Mary about this – not about this,’ I gestured at the charts, ‘I mean my feelings about Greg. She wasn’t sympathetic. She was even angry with me, as if it was some insult to her that I wasn’t immediately accepting that my husband had been having an affair and had had a crash with the woman he really loved.’

‘No one’s saying that,’ said Gwen. She looked at my charts almost with an expression of pity. ‘I don’t really know what to make of this.’ She took my hand. ‘I’m not an expert but I’ve heard that there are stages of grief and at the beginning it’s anger and denial. It’s completely understandable that you feel anger. I think the point of mourning is to get through that and reach some kind of acceptance.’

I pulled my hand away. ‘I know all of that,’ I said. ‘I read a piece about it once in Cosmo. And you know what I was thinking when I was doing all this crazy stuff with coloured stickers and ringing people up under false pretences? What would make it easy would be to find just one deleted email, just one scrap of paper in a pocket, that would show Greg had been having an affair. Or even just one occasion when he wasn’t where he was meant to be, or a missing afternoon when nobody knew where he was. Forget denial. Then I could just get angry and be sad, and my life would continue. There’s no trick to proving somebody’s having an affair. You catch them at it, just once. But how do you prove somebody’s innocent? What do you suggest?’

Gwen shook her head. ‘I don’t know,’ she said.

‘You have to do something like this,’ I said. ‘Something obsessive and excessive. You have to fill in all the gaps, then the gaps between the gaps so there’s just no space for this relationship. You know I went to see the police?’

‘Ellie, you didn’t!’

‘I told this woman officer I was convinced my husband wasn’t having an affair. She didn’t seem to believe me. I don’t think she thought it even mattered whether he was or not. The case was closed. This wasn’t something she wanted to hear about. But if I showed those charts to the police, do you think it would make a difference?’

Gwen frowned at the charts for a long time. ‘Honestly?’ she said.

‘Yes.’

‘This is amazing,’ she said. ‘Scary but amazing. I don’t think the police would pay much attention to it, but if they did, they might say, “Perhaps he was seeing the woman while he was doing other things. Perhaps she met him while he was buying his sandwich, perhaps she went with him in the car to his meetings. Or you might be right. Maybe they didn’t meet in that month. She could have been away and they were meeting up again on the day they crashed.”’

I took a deep breath. My first impulse was to be angry with Gwen, to shout at her and show her the door, but I stopped myself. She might have humoured me. Instead she had said what she really thought.

‘And if they said anything,’ Gwen continued, ‘it would be that you’re ignoring the only piece of evidence that really matters, which is that Greg and the woman died together. What in the end can you really say to that?’

I thought for a moment. ‘That it’s difficult to be innocent,’ I said. ‘And to prove you’re innocent is impossible.’