‘No!’ I leapt to my feet with a shout.

‘Turn it in the lock.’

I inserted the key. I jiggled it. Nothing. Twisted it again. The box opened with a snap. I had had the damned thing all along! I sank into the chair before the box, relieved.

Holmes yawned and ambled back to the settee, collapsing back onto it. ‘I shall leave you to it, Watson. I am a bit tired. Now, off to slumberland.’ He laid his pipe on an errant plate, stretched out, and closed his eyes.

I stared at the box.

What had my mother meant by this?

I opened it fully, not sure what to expect. A jewel? A keepsake? Another puzzle? But inside the box was nothing but two folded pieces of faded pink paper. I unfolded them and recognized my mother’s precise and beautiful hand. I read:

August 11, 1863, on the occasion of your eleventh birthday, my dear, darling John—

With luck you are reading this on August 11, 1873, and you are now twenty-one years old and ten years past my death.

I pray that you will forgive me for all that I am about to tell you and will understand my delay in doing so.

You no doubt suspected, and I will confirm, that my death was at my own hand. Please be assured there was nothing you or anyone could have done to dissuade me. It was not despair. As I write this, I face a terrible and incurable cancer, which over the next six months will consume me and put not only myself, but all our family through needless torture. I decided to spare us all.

I have chosen to shed my earthly bonds, but I trust, dearest son, that I will rest in the hands of a forgiving God and am now looking down at you with love. At eleven you were not ready, but I believe that as a man of twenty-one you will understand.

The second fact you must know is this. You were not responsible for the death of your twin. I know that you and Rosie argued that day at our picnic on the river, and that you left her and walked home, thinking your brother had stayed to look after her.

But when you discovered Harry at home in the bath, you thought you had made a terrible error leaving Rosie alone at the river. You ran back, but the poor child was already dead, drowned in the rapids, and you blamed yourself.

I must set the record straight. Your brother admitted the following to me yesterday. Harry was in fact with Rose when you left, as you thought. But what you didn’t know was that he saw her slip in the river and hit her head on a rock. He leaped in to save her, but the currents took her out of reach. Harry then saw her dashed against the rocks downstream and knew she was dead.