The Adventure of the Greek Interpreter 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. Mycroft, as Watson learns, does not have the energy of his younger brother and as a consequence is incapable of using his great skills for detective work: In spite of his inertia, the elder Holmes has often delivered the correct answer to a problem that Sherlock has brought to him.
Minor Sherlock Holmes characters their relationship. She also posed as John Watson's therapist and then later revealed herself to be Eurus Holmes, and at the end of the episode shoots John Watson with a handgun. In the last episode, "The Final Problem", it is revealed that Sherrinford is in fact a high-security complex on an island to house some of the most dangerous criminals in the world, including Eurus. With Eurus having simply shot John with a tranquiliser dart, Sherlock and John stage a scenario where Mycroft will admit Eurus's existence. Mycroft explains that Eurus was the youngest child, 8 years younger than Mycroft