"Let us reconstruct, Watson," said Holmes after half an hour of
silence. "I am not aware that in all our joint researches we
have ever had a case which was more difficult to get at. Every
fresh advance which we make only reveals a fresh ridge beyond.
And yet we have surely made some appreciable progress.

"The effect of our inquiries at Woolwich has in the main been
against young Cadogan West; but the indications at the window
would lend themselves to a more favourable hypothesis. Let us
suppose, for example, that he had been approached by some foreign
agent. It might have been done under such pledges as would have
prevented him from speaking of it, and yet would have affected
his thoughts in the direction indicated by his remarks to his
fiancee. Very good. We will now suppose that as he went to the
theatre with the young lady he suddenly, in the fog, caught a
glimpse of this same agent going in the direction of the office.
He was an impetuous man, quick in his decisions. Everything gave
way to his duty. He followed the man, reached the window, saw
the abstraction of the documents, and pursued the thief. In this
way we get over the objection that no one would take originals
when he could make copies. This outsider had to take originals.
So far it holds together."

"What is the next step?"

"Then we come into difficulties. One would imagine that under
such circumstances the first act of young Cadogan West would be
to seize the villain and raise the alarm. Why did he not do so?
Could it have been an official superior who took the papers?