‘And you’re right, Max. I am changed from the woman I used to be. And the woman I wanted to be. And it’s not because of what happened to Justin. It’s not because of what some mindless little thug did to him in some bar. It starts long before then. It starts when his father left us. I always thought that being a single parent would make me a better person. All that sacrifice and caring for someone who depends on you for everything. But it didn’t make me a better person. It made me harder. I find I care less about the outside world. Sometimes all my compassion is reserved for my child. Maybe that’s what’s wrong with me. Do you know what I mean?’

Of course I knew what she meant.

I watched her dispose of the blood-gorged syringe.

‘You need to walk Dasher,’ I said. ‘That’ll cheer you up.’

I stayed to watch them prepare for that very first walk. I watched them put Dasher into his bright yellow harness with its blue Guide Dogs logo. I watched them double- and triple-checking that the lead was correctly attached to the collar, and that they had enough poo bags, hand wipes and dog treats for twenty walks. And I smiled because all those preparations would become second nature before they knew it. Then they stepped out into the quiet street, bound for the open pastures of Highbury Fields, Whitestone and Justin talking nervously as Dasher wore his reassuring grin between them.

By the end of the street, I could hear their laughter and I could see the spring in their step.

Dogs can break your heart. But they can heal it too.

Then I drove across town and picked up the motorway at Brent Cross, flexing my left arm as I followed the GPS to Summerdale Psychiatric Hospital.

It was feeling better already.

‘Summerdale is increasingly focused on promoting mental health rather than simply responding to mental illness,’ the hospital’s Director of Nursing told me. She was a tall woman nearing the end of her career, and in her classless accent I could detect just a hint of Australia, the country she had left a lifetime ago. ‘We want people to get well and stay well,’ she said. ‘We believe change is possible. But we also provide mental health care for long-term residents. Like Tommy Defarge.’

I looked out across the grounds. It could have been a private school, or a country hotel.

‘So in the past, people would come here and stay here,’ I said. ‘But now they receive care and then they leave. Is that about right?’

She nodded.

‘But the young man you’re interested in – no longer so young, but then neither am I – is one of our long-term residents. You have to understand, Detective, that there was a stigma about mental health when Tommy Defarge was born.’

‘He’s never going to get better?’