The intruder was a man of commanding presence, squarely built, with a very round head and a powerful jaw. He might have been about forty-five years of age.

‘I must beg your pardon,’ said the stranger, advancing into the room, hat in hand. ‘I found your outer office empty and this door open, so I ventured to intrude. This is Blunt’s International Detective Agency, is it not?’

‘Certainly it is.’

‘And you are, perhaps, Mr Blunt? Mr Theodore Blunt?’

‘I am Mr Blunt. You wish to consult me? This is my secretary, Miss Robinson.’

Tuppence inclined her head gracefully, but continued to scrutinise the stranger narrowly through her downcast eyelashes. She was wondering how long he had been standing in the doorway, and how much he had seen and heard. It did not escape her observation that even while he was talking to Tommy, his eyes kept coming back to the blue paper in her hand.

Tommy’s voice, sharp with a warning note, recalled her to the needs of the moment.

‘Miss Robinson, please, take notes. Now, sir, will you kindly state the matter on which you wish to have my advice?’

Tuppence reached for her pad and pencil.

The big man began in rather a harsh voice.

‘My name is Bower. Dr Charles Bower. I live in Hampstead, where I have a practice. I have come to you, Mr Blunt, because several rather strange occurrences have happened lately.’

‘Yes, Dr Bower?’

‘Twice in the course of the last week I have been summoned by telephone to an urgent case—in each case to find that the summons has been a fake. The first time I thought a practical joke had been played upon me, but on my return the second time I found that some of my private papers had been displaced and disarranged, and now I believe that the same thing had happened the first time. I made an exhaustive search and came to the conclusion that my whole desk had been thoroughly ransacked, and the various papers replaced hurriedly.’

Dr Bower paused and gazed at Tommy.

‘Well, Mr Blunt?’

‘Well, Dr Bower,’ replied the young man, smiling.

‘What do you think of it, eh?’

‘Well, first I should like the facts. What do you keep in your desk?’

‘My private papers.’

‘Exactly. Now, what do those private papers consist of? What value are they to the common thief—or any particular person?’

‘To the common thief I cannot see that they would have any value at all, but my notes on certain obscure alkaloids would be of interest to anyone possessed of technical knowledge of the subject. I have been making a study of such matters for the last few years. These alkaloids are deadly and virulent poisons, and are in addition almost untraceable. They yield no known reactions.’

‘The secret of them would be worth money, then?’

‘To unscrupulous persons, yes.’

‘And you suspect—whom?’

The doctor shrugged his massive shoulders.