It happened that a locked and deadly secret played a key role in each of the two cases – and a puzzle of my own – which presented themselves to us that fateful month. So complex were these gruesome mysteries, and so tragic the outcomes, that it took the genius, bravery and resourcefulness of Sherlock Holmes to solve them, and a certain transparency of my own to tell of them now. My hesitation in doing so earlier will become apparent, perhaps only at the very last.

Our rooms faced east on Baker Street, and on that day I had closed the heavy curtains against the morning sun, leaving the windows behind them cracked open and myself sitting in half light, too hot even to read. Despite my claim that I had become inured to the heat during my army years in India and Afghanistan, I suffered mightily.

I contemplated asking Mrs Hudson for an iced lemonade, but the poor woman was no doubt prostrate on her bed from the heat. A paper fan lay next to me, useless in this oven. My ruminations were interrupted when the postman arrived with a small rectangular package, addressed to me and postmarked Edinburgh.

I welcomed the distraction.

The package was perhaps half the size of a shoebox, and heavy. The sender was a mystery, one ‘E. Carnachan’. I unwrapped it, only to find a smaller box inside covered by another layer of brown paper wrapping, this one stained, faded and tied with string. A note had been slipped under the string.

I removed and unfolded it, my hands sticky with sweat. The handwriting was old-fashioned and feminine, shaky, perhaps with age or illness. I read:

Dr John Watson—

You do not know me, but I am Elspeth Carnachan, a name you may not recognise. Carnachan is my married name, but I was born a Watson and your father’s half-sister. Your father and I parted ways while still in our youth, and I was effectively erased from the family history before you were born. It is doubtful that he ever mentioned me to you.

Your mother, bless her kindly soul, maintained contact despite your father’s ill opinion of me. You can confirm my familial connections, if you so choose, by public records in Edinburgh.

I am an old woman now and consumption will soon take me. In clearing my affairs, I came upon something in my attic last week that was meant for you. It is enclosed.

When you were only eleven your mother gave it to me for safekeeping with the direction to give it to you on your twenty-first birthday. She was in perfect health when she entrusted it to me, and so I wondered why.

By a strange twist of fate, she died only two days later. Perhaps she was prescient? We shall never know.

I placed it in my attic, intending to deliver it at the designated time. But life intervened and it slipped my mind.