The Adventure of the Greek Interpreter

"The Adventure of the Greek Interpreter", one of the 56 Sherlock Holmes short stories written by British author Sir Arthur Conan Doyle, is one of 12 stories in the cycle collected as The Memoirs of Sherlock Holmes. The story was originally serialised in Strand Magazine in 1893. This story introduces Holmes's elder brother Mycroft. Doyle ranked "The Greek Interpreter" seventeenth in a list of his nineteen favourite Sherlock Holmes stories.
On a summer evening, while engaged in an aimless conversation that has come round to the topic of hereditary attributes, Doctor Watson learns that Sherlock Holmes, far from being a one-off in terms of his powers of observation and deductive reasoning, in fact has an elder brother whose skills, or so Holmes claims, outstrip even his own. As a consequence of this, Watson becomes acquainted with the Diogenes Club and his friend's brother, Mycroft.