These were the precise conditions under which my friend was most likely to turn towards artificial stimulants. While I longed to escape, or to pursue my mysterious silver box with another try at a locksmith, I resolutely remained with him, and vigilant. I busied myself with notes on old cases, a couple of cheap yellow-backed novels, and the newspapers. As the days passed, even the mystery of my silver box seemed to fade in the atmosphere of indolence and malaise that had settled into 221B.

It was upon the fifth day of this self-imposed confinement that I chanced upon a notice in The Times that Professor Richard Wyndham of Cambridge had announced the engagement of his daughter Miss Odelia Wyndham to Mr Frederick Eden-Summers, eldest son of the Duke of Harbingden. An unsmiling portrait of the young lady appeared next to the notice. Holmes’s response was a dismissive wave of the hand and silence. Nothing I could say would entice him to dress or to leave our rooms and go for a walk or any other activity.

By the next day, Thursday, I could stand internment with my morose companion no longer and ventured out into the sweltering city with my mysterious silver box to Chubb’s Lock Company, the second of the locksmiths that he had recommended.

Two hours later I returned, after having received the identical response I had gotten earlier from Mr Boobbyear and having been charged an exorbitant price for this useless information. The infernal box was secured with a trick lock and apparently was impregnable, except by destroying it. As I entered into the foyer, Mrs Hudson greeted me with a worried look. ‘What is it?’ I asked.

‘Listen.’

I could hear the strains of Holmes’s violin playing a racing, frenetic melody.

‘Well, better that than his recent tuneless atrocities,’ I remarked.

‘But it has been going on since you left. Some two and a half hours, Doctor. With no pause. None at all!’

That set me back. ‘Since immediately after I left?’

‘No, about ten minutes after.’

That could mean only one thing. I raced up the stairs.

Holmes was standing silhouetted in the window, his nightshirt billowing in the faint breeze, playing in a frenzy of excitement.

‘Holmes!’ I cried.

He did not hear me but continued playing. The piece reached a crescendo and ended. He put down the violin with a flourish and waved his bow in the air.

‘I have done it!’ he shouted.

‘What?’

‘I have memorized all six of Bach’s Sonatas and Partitas! Without a flaw! I recreated them in my mind, retrieving it page by page. I could see it!’

His eyes were feverishly bright. His face was damp with perspiration, and his whole body vibrated with excitement.

‘In two hours? How do you know it was perfect?’

‘Memorization! I review it once! I have it here,’ he cried, tapping his forehead with his bow.

‘Sit down, Holmes. You are in a kind of mania!’