John Watson was honourably discharged from the army as a captain.
In BBC’s Sherlock, Watson returns to London after serving in Afghanistan with the Royal Army Medical Corps. His military service explains both his medical expertise and his familiarity with combat, weapons and military procedures. The series presents him as a doctor and former army officer who has been invalided home after suffering physical and psychological effects connected with the war.
The rank is sometimes confused with lieutenant or major because Watson is frequently described simply as a military doctor, and the show does not make his rank central to every episode. However, his character biography explicitly identifies him as a captain when he leaves the army. Sherlock also recognizes Watson’s military background and uses his experience when cases involve danger or restricted locations.
Watson’s army history is a modern adaptation of Arthur Conan Doyle’s original character, who was a military surgeon wounded in Afghanistan. The television series preserves that essential background while updating the conflict and placing Watson in the Royal Army Medical Corps.