Watching The Naval Treaty in The Adventures Of Sherlock Holmes my mind wandered to Woodrow Wilson and one of his 14 points which was 'open covenants openly arrived at', something he was trying to get into the Treaty of Versailles. It was against just such things as this, a secret Naval Treaty between the British and the Italians, something the foreign offices of any number of European powers would have loved to have gotten their hands on in those days before World War I.
The document, written in French, because in Europe that was the universal language of diplomacy, is entrusted to David Gwillim, a clerk in the foreign office. Gwillim is in that position because of his uncle is the foreign minister. One night the document is stolen and Gwillim facing personal ruin as well as the potential crisis in foreign relations the United Kingdom could suffer calls on a friend of a friend. Gwillim went to school with David Burke as Dr. John Watson and we know who his friend is. Jeremy Brett is the one man in the country who could untangle this potential crisis and keep it from coming to a head.
Given the limited number of characters in a short story, let alone one of Arthur Conan Doyle's stories about the master sleuth, we only have a limited number of suspects. It was obvious to me who the guilty party was almost from the beginning. Yet Conan Doyle's stories are mind games, the treat in reading and watching a dramatization is to see how Sherlock Holmes's mind works.
We don't have secret treaties any more, not because nations wouldn't like to have them. But in this the day of the internet, such secrets can't be kept for too long. What Wilson couldn't get by treaty, technology has forced upon nations.