‘Excuse me, gentlemen,’ said a voice behind us. We turned to see a charismatic and darkly handsome young man in his late twenties, with a swath of thick, black hair and the chiselled features of a theatre performer. ‘I am Cosimo Fortuny. We do not allow visitors in this laboratory, for the very reason you have just witnessed. Who are you and what is your business here?’ He had a mellifluous, cultured voice.

‘Ah, Mr Fortuny!’ exclaimed Holmes. ‘I have read of your work on the effects of rapidly alternating currents on various gases! A pleasure to meet you. I see you are trying Geissler tubes of various shapes and diameters. Why, I wonder?’

If Fortuny was impressed, it was not apparent. ‘What is your business, sir?’ he asked coldly.

Holmes gave our names again. ‘We are up from London on personal business that concerns Mr Vitale. Nothing to do with your research.’

‘Then how do you know of it?’

‘Mr Holmes follows such things,’ I said. ‘Much as other men follow football. Those Hiburnians, quite something, eh?’

Fortuny laughed. ‘Ah, all right! An amateur scientist. I know your kind. You doubtless have some arcane specimens of something in a glass case. Perhaps even a small Bunsen burner in your sitting-room. Go ahead, Leo. Remove yourselves to the hall, please.’

I sensed Holmes about to make a stinging retort and pulled him through the door.

In a moment, we stood facing Leo Vitale in the hallway. There was something awkward about the thin young man, something not at ease. It was as if he had only newly inhabited this body and still didn’t know how it worked. His gestures were stilted, self-conscious. But when he smiled, which was rare, this awkwardness melted away and the fellow did, I suppose, have a certain uncultivated charm.

‘How can I be of help, Mr Holmes?’ he asked.

‘Mr Vitale, I have been called in by Miss Wyndham’s family,’ said Holmes. ‘They are worried about her disappearance. May I see the bottom of your left shoe?’

I looked down at Vitale’s shoes. They were of good quality but worn. What on earth was Holmes on about?

The fellow hesitated, then raised and twisted his foot to display the sole, on which a large patch was evident. Holmes smiled at this.

Vitale frowned, then understanding dawned. ‘My footprint?’ he mused. ‘You are a detective. But where …?’ His eyes flicked back and forth, and he put a hand to his forehead. ‘Oh, the tree! Her house!’ He coloured violently. ‘How careless of me,’ he mumbled. ‘But she hasn’t truly disappeared.’

Holmes smiled at the boy’s quickness. ‘I am aware. We have just come from her. I am not here to censure, Mr Vitale. Rather to understand who might have sent a threatening letter to Miss Wyndham.’