Strangely, the sweet face of my long-dead mother appeared in a dream. She was frowning and waved an index finger at me. In the moment, I was much shorter than she, a small boy. She was admonishing me, but I could not hear the words, try as I might to understand her. This image faded, blurred and reappeared. Now my mother was slightly older, but her face looked wavy, eerily tinged with blue green as if underwater, and her eyes were bulging.

I awoke with a start as our train was pulling into King’s Cross. My mother’s image stayed with me, and I was left with a feeling of dread. When I was eleven my mother had drowned, and the circumstances of her death were cloudy. I never could believe suicide. The tragedy had scarred our family, and my elder brother Harry had never recovered.

The brakes of the train squealed loudly as we slowed into the station, and I turned sharply away from these thoughts. I had rarely been troubled by nightmares.

In minutes we were back at Baker Street. Holmes sent a cable to the Wyndhams, explaining that Odelia was safe, and they could communicate to her via our address. I retired to my room and checked that my silver box was safe in my desk drawer.

As I closed and locked the drawer, I suddenly saw my mother’s drowned face again. I shuddered and blinked it away, feeling the full effect of two tumultuous days and the blinding heat. I was exhausted. Tomorrow, I would take my box to someone Holmes recommended.

The next morning, I awoke to find Holmes had already breakfasted and was out on an unknown errand. It had rained overnight but the oppressive heat had turned the summer showers into a steamy downpour. I sat over coffee, contemplating the small silver box which I had freed from my desk drawer. I toyed with the slender metal bands that braided decoratively around it, culminating in that mysterious lock. Would moving them trigger the lock in some way?

I heard Holmes arrive downstairs and tucked the bonny thing into my dressing-gown pocket.

He joined me at the breakfast table. His eyes raked over me in that disconcerting fashion. ‘There is Chubb’s over near St Paul’s. They were tasked with locking up the Koh-i-Noor Diamond during the exhibition. You could try there,’ he said.

‘What?’

‘Most famous locksmith in London. If he cannot help you, then Boobbyer, 14 Stanhope Street, the Strand, might.’

‘Boobbyer? Odd name!’

He smiled. ‘Probably an attempt at anglicization of a French word. Bobbière, or something.’

‘Holmes, how on earth do you know of—’

‘Simple research, Watson!’

‘No! You are not a mind reader! How did you know that I was thinking of the box?’

‘Bulge in your dressing-gown pocket. Cigar-cutter on the table. You don’t smoke cigars in the morning.’

‘But I might.’