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Holmes and Watson are returning from Manor House to London by train, discussing a recently solved crime and the death of a certain Lord Huxley. At home, Holmes receives a letter from his older brother Mycroft, an employee of the Foreign Office and a member of the so-called Diogenes Club of Silent Men, asking for a meeting. Holmes notes that Mycroft is far more gifted in deduction than he is.

The detective understands that the cipher is the property of criminals - members of a large criminal community. Holmes decides to expose this community, which he began to do in the following days. Together with Watson, he decides to follow Sebastian Moran - a retired colonel, an excellent marksman and a respected man who became a dangerous criminal. To do this, they visit the gambling club "Bagatelle", where Moran was a regular. In the club, Holmes receives a note asking for a meeting. The detective leaves, and Watson remains to watch Moran.

Watson is going to finish Holmes's case and decides to protect the young aristocrat Ader. He dresses up as a Catholic priest and goes to the Bagatelle Club to watch Ader. Unfortunately, Watson is recognized by a visitor to the club, Mr. Murray, who was once treated by him. At this time, in the hall of the club, the marker Price reminds Ader of his debt to Colonel Moran - a win of 420 pounds, and advises him to give the money back, but Ader refuses, stating that the colonel played unfairly and the winnings were returned by the loser. Watson goes after Ader in a cab, but he gets out of it already in civilian clothes. This was the second failure. The third failure was that Watson was remembered by the butler when he wanted to know if Ader was at home. The owner locked himself in the room to calculate his card debts. Some time later, he was found shot.