‘Who was the other man with a key?’

‘The senior clerk and draughtsman, Mr Sidney Johnson. He is a man of forty, married, with five children. He is a silent, morose man, but he has, on the whole, an excellent record in the public service. He is unpopular with his colleagues, but a hard worker. According to his own account, corroborated only by the word of his wife, he was at home the whole of Monday evening after office hours, and his key has never left the watch-chain upon which it hangs.’

‘Tell us about Cadogan West.’

‘He has been ten years in the Service, and has done good work. He has the reputation of being hot- headed and impetuous, but a straight, honest man. We have nothing against him. He was next Sidney Johnson in the office. His duties brought him into daily, personal contact with the plans. No one else had the handling of them.’

‘Who locked the plans up that night?’

‘Mr Sidney Johnson, the senior clerk.’

‘Well, it is surely perfectly clear who took them away. They are actually found upon the person of this junior clerk, Cadogan West. That seems final, does it not?’

‘It does, Sherlock, and yet it leaves so much unexplained. In the first place, why did he take them?’

‘I presume they were of value?’

‘He could have got several thousands for them very easily.’

‘Can you suggest any possible motive for taking the papers to London except to sell them?’

‘No, I cannot.’

‘Then we must take that as our working hypothesis. Young West took the papers. Now this could only be done by having a false key—’

‘Several false keys. He had to open the building and the room.’

‘He had, then, several false keys. He took the papers to London to sell the secret, intending, no doubt, to have the plans themselves back in the safe next morning before they were missed. While in London on this treasonable mission he met his end.’

‘How?’

‘We will suppose that he was travelling back to Woolwich when he was killed and thrown out of the compartment.’

‘Aldgate, where the body was found, is considerably past the station for London Bridge, which would be his route to Woolwich.’

‘Many circumstances could be imagined under which he would pass London Bridge. There was someone in the carriage, for example, with whom he was having an absorbing interview. This interview led to a violent scene, in which he lost his life. Possibly he tried to leave the carriage, fell out on the line, and so met his end. The other closed the door. There was a thick fog, and nothing could be seen.’

‘No better explanation can be given with our present knowledge; and yet consider, Sherlock, how much you leave untouched. We will suppose, for argument’s sake, that young Cadogan West had determined to convey these papers to London. He would naturally have made an appointment with the foreign agent