The Adventure of the Second Stain

Trelawney Hope, Secretary for European Affairs, reaches out to Holmes. A very important letter was stolen from Hope’s safe and he wants Holmes to track it down. The letter’s content was of political importance and Holmes's three main suspects are international secret agents. However, one of them, Eduardo Lucas, has been found murdered. Is there some connection between the two crimes and will Holmes connect the dots?
"The Adventure of the Second Stain" is a part of "The Return of Sherlock Holmes".