‘A twin sister!’ He took some time lighting the pipe. ‘Ah, I am sorry for this. But your mother’s death, Watson – was it by her own hand, then?’ he asked gently.

‘We were never sure.’ I felt sick thinking of it. I kept running my fingers over the engraving.

‘Was she despondent? Hysterical? Grieving, perhaps.’

‘No. This was several years after Rose. My mother was not subject to moods, Holmes.’

‘Nor are you, most of the time.’ He paused. ‘This box may have been long in the planning. She must have given you the key. Think! Was there any unique, decorative, mysterious item of metal in anything she gave you before this?’

‘I was only eleven when she died. She would not have entrusted a key to a small child!’

Open the box, a voice inside me shouted. No, don’t open the box, came another. I closed my eyes. My mother’s face, smiling at me. Then my mother’s face, still and white, eyes bulging.

‘Watson, I wager your sterling qualities were well evident to your mother, even during your early childhood. Do you still have that small box of mementos from your youth?

Ah, the one with the wooden soldiers that I had mentioned to Knut Lossop? But why was he asking now, I wondered?

‘Is there nothing metal, oddly shaped, in that box? Nothing that could be a key?’

I snapped back to the present. ‘Well, I – oh!’ There was one thing. It was a small ornate clock puzzle that had never worked to keep time. I had forgotten it when prompted for a ‘treasure’ at Lossop’s and was later glad of it. It was my dearest possession, my mother’s last Christmas gift to me, eight months before her death. I said as much to Holmes.

I limped upstairs, retrieved the small cigar box of childhood treasures, and set the gilded clock puzzle on the table near the silver box. Holmes arose from the settee and faced me across the table, pipe in hand.

‘Go on, Watson. The clock puzzle first.’

I sat down before the box, and because he was so insistent, reluctantly took up the little clock. It was ornate and odd, constructed of several pieces. It had never kept time and was decorative only. But it had pleased me inordinately. I unpacked its pieces. To my amazement, one of them was silver, and filigreed in a style just like the box. I held it up and it was the right size to match the small keyhole below the box’s lock.

This was too damnably easy.

‘I—my God, you may be right!’ I said. I touched it to the edge of the keyhole. ‘It looks like it fits!’

‘Good,’ said Holmes ‘Then your mother planned this for you, as I thought.’ In a sudden move, he snapped the lid of the silver box down, and the lock clicked shut once again.