So I sat on his bed and I waited. Dasher sighed and closed his eyes. Justin’s hand soothed his head.

‘After what happened, everything changed,’ he said. ‘I felt like the future I was expecting had been taken away. That caused all those feelings. Depression. Self-pity. I know it’s pathetic.’

‘It’s not pathetic.’

‘But I’ve had a chance to think now. And I know I want to be like you and my mum – I want to do what you do.’

I did not speak.

‘I want to make a difference,’ he said. ‘And I wanted you to tell me honestly – is that just stupid?’

I shook my head.

‘No, Justin, it’s not stupid. The Met has thirteen thousand civilians working with thirty-one thousand officers. They’re all sorts of people, with all sorts of skills. What happened to you – that doesn’t have to stop you. But first you have to get out of this room and out of this house. It’s always going to be hard, Justin. It’s already going to be harder for you than I can imagine. But that’s why Dasher is here.’ I didn’t know what else to tell him. I stood up. ‘OK?’

‘OK, Max. And thank you.’

I went downstairs leaving the sleeping Dasher with him.

‘Your son thinks we make a difference,’ I told Whitestone.

‘And you don’t,’ she said. ‘Because you went to see Billy’s family and now you wonder if it’s worth it. But we all wonder if it’s worth it, Max. You have to stop thinking that makes you a bad policeman. It doesn’t. It makes you human.’ She gestured at my arm. ‘Let me see that elbow.’

‘It’s all right,’ I said.

She gestured impatiently. I rolled up the sleeve of my shirt.

My left arm was a mass of black and purple bruising from my bicep to my wrist. The point of my elbow had swollen up to the size of a ripe apple.

‘Olecranon bursitis,’ Whitestone said, prodding it. ‘There’s a little sac of fluid around the joints. The bursa. It helps movement. But it fills with blood if it has a trauma. That’s what has happened here. It needs to be drained.’

She came back from the kitchen with a first-aid box, took out a disposable hypodermic needle and eased it into the livid red lump that was now my elbow.

‘You think I’ve become a heartless old cow,’ she said, as we both watched the needle fill with blood that was closer to black than red. The lump on the tip of my elbow began to shrink.

‘That’s a bit harsh,’ I said. ‘You’re not that old.’