Dr Holt scrubbed at the clouded-up windscreen with his sleeve. ‘This is a four-wheel drive. I bought it after those few bad winters a while back, when I couldn’t get to work.’

We were crawling down the frozen M25, hardly any other vehicles braving the trip. I could see almost nothing, just gusts of wind that from time to time cleared the freezing fog. I had explained all I knew – my original plan, the revisions I’d made to it when I learned about the baby, my growing doubts about Nick, why I thought Suzi was imprisoned inside the house.

It turned out he and Suzi had seen each other a few times, and I detected a tenderness towards her. She was the type of woman men wanted to take care of. He admitted, ‘I was worried about Suzi. The things she said about Nick, how possessive he was, it sounded like coercion.’

‘You don’t know the half of it.’ I told him what I’d learned, that Nick watched Suzi through their alarm system, that he reset the heating and door codes to wrong-foot her, that he played music remotely to make her think she was going mad. Dr Holt’s hands clenched on the steering wheel.

‘Yet just because he doesn’t hit her, no one would think it’s abuse,’ he muttered. I deduced from this he was not just thinking of Suzi.

‘I think he might have moved on to more physical threats now,’ I said, thinking of the cellar.

‘Pregnancy is when most instances of domestic abuse get worse,’ he said, glancing anxiously at me. ‘Eleanor – one of the other doctors died a few days ago. James Conway – he knew your husband, I think. He knew about Patrick and Suzi.’

‘He was going to blackmail her,’ I said. I didn’t explain how I knew this.

‘Do you think that maybe Nick . . .’

I didn’t answer. Now was not the time to open that particular can of worms. After a moment of silence, he said, ‘I don’t want to miss the turn-off – tell me when we’re close.’

We almost did miss it in the gloom of the whirling snow, then had to take the turn too fast. I held on to the side handle, white-knuckled. Then we were bumping down the narrow, dark lane, the dazzle of the motorway suddenly gone, the car rocking over potholes left from earlier freezes. Ivy Cottage appeared, forlorn and snow-covered. And someone was at my door, hammering on it, desperate. Suzi! I saw she wore only thin clothes, and had no shoes on. Her feet were bare and red in the lights from the car.

‘Stop!’ I shouted, grabbing for the wheel without thinking. ‘It’s her!’