|Name:||Miss Mary Morstan, (married name, Mrs. Mary Watson)|
|Relationships:||John Watson (husband), Captain Morstan (father, deceased), Sherlock Holmes (family friend)|
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Mary Morstan is the wife of John Watson.
She was possibly born in India; her mother died shortly after her birth, and her father was the senior captain of an Indian regiment. She had no other relatives in England, and was sent to boarding school in Edinburgh. When she was seventeen, her father disappeared under strange circumstances. Eleven years later, Sherlock Holmes would solve this mystery, and Watson would write about it, calling it "The Sign of Four." During the events chronicled in this story, Mary and Watson fall in love, and at the conclusion of the case, Mary accepts Watson's proposal of marriage. They get married in 1889.
Mary is blonde, with blue eyes. She is not described as being beautiful, but as having a sweet expression and sympathetic eyes, which hint at her character as a warm and supportive person. In "The Man with the Twisted Lip," Watson writes "Folks who were in grief came to my wife like birds to a lighthouse." Although generally not fond of women, Holmes is quite complimentary towards Mary, particularly her intelligence and potential aptitude for detective work. 
In his final note to Watson before vanishing at Reichenbach Falls in 1891, Holmes asks Watson to "give [his] regards to Mrs. Watson." Three years later, upon his return, he expresses his condolences on Watson's recent loss; Watson writes, "In some manner he had learned of my own sad bereavement."
Arthur Conan Doyle
Although there is no ACD canon as to the cause of Mary's death, fanon generally assumes that she died in childbirth, along with her child, although sometimes it is tuberculosis or some other wasting disease. There are some indications in canon that Watson may have married again, but no specific woman is ever mentioned.
-- something here about the 'James' incident
Portrayed by Yekaterina Zinchenko.
Portrayed by Jenny Seagrove.
Interestingly, although the earlier series of Granada's Sherlock Holmes are almost fanatically canon-compliant, in their version of "The Sign of Four," Watson and Mary do not get engaged at the end of the episode.
Portrayed by Kelly Reilly. Ritchie-verse Mary is still a governess, but not an orphan, as Watson mentions having plans to meet her parents. She was engaged once before, but her previous fiancé died.
Mary apparently met and became engaged to Watson without ever meeting Holmes; her first encounter with Holmes is at a restaurant for a formal dinner, not in his sitting room as a client. Watson apparently hasn't told Holmes much about Mary either, as he deduces that she is a governess.
As Watson's belongings are being moved out of 221b Baker Street, he refers to a trunk full of notebooks dismissively as "scribbles." Mary responds with enthusiasm that they are his adventures and she'd like to read them. This has led to movieverse fanon that Mary is the one who helps edit Watson's stories, bringing them up to a professional level, or just encouraging him to get them published. 
At the end of the first movie, Mary is wearing a ridiculously massive diamond ring; she was given the ring by Watson, who was given it by Holmes, who stole it from Irene Adler.  The OT3 or even OT4 implications of this chain of re-gifting have not gone unnoticed in fandom. (Watson himself calls it "the ring he's given us.")
Occasionally in post-movie fic, Irene returns to get her diamond back, and then there is femslash.
- Mary Morstan tag @ AO3 - Sherlock Holmes (2009) fic only
- "Oh, hello my dear." John/Sherlock/Mary OT3 meta by recessional
- Arrangements by radial_symmetry: Holmes and Mary Morstan have a little chat, and reach an agreement.
- Marriage á Trois by Sorrel - Holmes/Mary/Watson OT3, which is also a great character study of Mary.
Portrayed by Amanda Abbington. Introduced in the third series premiere "The Empty Hearse", taking place two years after Sherlock's faked death. Mary is a nurse who has been dating John for six months prior to Sherlock's return. Sherlock reveals himself to John just as John is about to propose to Mary. Mary takes a shine to Sherlock and encourages John to go see Sherlock despite his anger.
Sherlock and Mary have a very good relationship in the BBC-verse. She is able to tell when Sherlock is lying, calling him out on it and getting him to admit the truth. Sherlock is incredibly protective of John and Mary's relationship, scaring off her pining ex-boyfriend and doing a great deal to plan their wedding. Sherlock accidentally reveals Mary is pregnant when he makes a vow to protect Mary, John and their baby.
Mary is revealed to be a former CIA assassin with the initials A.G.R.A, taking the name Mary Morstan from a stillborn five years prior to the events of the series. She is being blackmailed by Charles Augustus Magnussen, whom Sherlock is investigating. Sherlock walks in on Mary about to assassinate Magnussen and she non-fatally shoots Sherlock in order to preserve her secret. John is horrified and disgusted by the lies of his pregnant wife, but Sherlock realizes she could have easily killed him and agrees to get Magnussen's documents back for her. John does not talk to Mary for months, but eventually forgives her, saying, "the problems of your past are your business. The problems of your future are my privilege".
Mary is an incredibly polarizing figure in the BBC Sherlock fandom, especially following the revelation of her past and her shooting of Sherlock.
- The Progress of Sherlock Holmes by ivyblossom contains an unflattering, but not unsympathetic, portrayal of Mary
- Entries tagged Mary Morstan on the BBC Sherlock kink meme
- Works tagged Mary Watson nee Morstan at The Sherlock Holmes Kink Meme Archive
- Entries tagged character: Mary Morstan on furtherinterest, a community devoted to the female characters of BBC Sherlock
- Works tagged Mary Morstan on the AO3
- In the final scene of SIGN Holmes says of Mary: I think she is one of the most charming young ladies I ever met, and might have been most useful in such work as we have been doing. She had a decided genius that way: witness the way in which she preserved that Agra plan from all the other papers of her father. The Sign of Four on Project Gutenberg.
- Actually this may be ACD fanon as well, which explains (on a Watsonian level) why the quality of some of the stories goes downhill after Mary's death.
- Who stole it from a maharajah. Hopefully he won't want it back.