Mary Watson (née Morstan) is the wife of Dr John Watson.
Shortly before his proposal to Mary, Watson arranged a dinner with her and Holmes to introduce him to his fiancée, a meeting which Holmes had been desperately trying to avoid. Initially Mary was friendly and very interested in Holmes' skills, admitting that she was a devotee of detective novelists such as Wilkie Collins and Edgar Allan Poe. She is sceptical, however, about the protagonists' abilities to make such grand assumptions from such tiny details. Holmes argues against her, that in fact the smallest details are sometimes the most important. To prove his point, he uses Watson's cane – a rare African snakewood hiding a blade, which was awarded to some veterans of the Afghan War – to deduce that he was a decorated soldier, and from that that he is strong, brave, neat, and a man of action. Then, in a barb against Watson, deduces from a boxing match ticket stub in his pocket that Watson is a bit of a gambler, and warns Mary to watch her dowry because he's lost the rent more than once.
Mary nonetheless remains unconvinced because of the pair's long friendship, and things go sour when against Watson's advice she insists he examine her. Holmes starts off by deducing that she is a governess and that her student, who is tall for his age, flicked ink at her that day, and that her level-headed response persuaded the lady of the house to let her borrow her jewellery. He next realises from a missing ring that she had previously been engaged; however, he upsets her by deducing she left her fiancé for money, when in truth he had died. Angrily, she throws her drink in Holmes' face and leaves the restaurant, followed by Watson.
Despite this bad first impression, Mary sees how much Holmes cares for Watson, when he pretends to be a doctor in order to visit Watson whilst on the run. In "Sherlock Holmes: A Game of Shadows" she seems to think better of Sherlock, and works with Mycroft in order to take down Moriarty.