Mary Elizabeth Watson (formerly Morstan, born Rosamund Mary) is the identity used by a former assassin.
She worked as a nurse in the same clinic as her husband John Watson, whom she met sometime after his best friend Sherlock Holmes' supposed death. She and John have a daughter named Rosamund Mary. She was killed by Vivian Norbury when trying to save Sherlock from being shot.
Mary's past is a little unclear; however, it is known that she was born Rosamund Mary and that she used to be a trained assassin who went rogue and changed her identity in order to hide from her past. Sherlock suspects that while her accent is English, she may not have been born British. Approximately five years before meeting John Watson in 2008, she chose the name "Mary Elizabeth Morstan" from the gravestone of a stillborn child, in order to hide her true identity, and claimed to be an orphan in order to cover for the fact that she had no family.
They had been together at least six months before John proposed.
Shortly after they first met Sherlock performed a series of deductions about Mary, which revealed that she was an only child, that she was clever, a nurse, linguist, short sighted, romantic, had a cat, wore size 12, was a disillusioned Liberal Democrat, bakes, had a secret tattoo and an appendix scar, and that she was a "liar". However, Sherlock did not reveal any of these deductions to anyone.
John planned to ask Mary to marry him shortly before Sherlock's return, he took her for a meal out and brought a ring, however the meal was interrupted by Sherlock just as John was about to ask her. When John finally saw that Sherlock was alive the horror on Mary's face was obvious, as she knew just what Sherlock's "death" put him through. Despite being together for less than two years, Mary was more of a serious girlfriend than the others girlfriends John had and he proposed to her soon after the interrupted meal.
Mary married John on the 18th May, at St, Mary's Church in Sutton Mallet, Somerset. The wedding was attended by among others, Greg Lestrade, Molly Hooper, Tom, Mrs Hudson, General Sholto and Sherlock who was John's best man. Her wedding was tumultuous and she received a possibly threatening telegram from CAM (who could be Charles Augustus Magnussen). Her first dance with John as a married couple was a waltz written by Sherlock. She was revealed to be pregnant during one of Sherlock's speeches at the wedding.
The truth about her past was revealed to some extent during Sherlock and John's investigation of Charles Augustus Magnussen. Under a threat from Magnussen, she went to his office intending to kill him, however she was discovered by Sherlock whilst he and John were breaking into the place. Knowing that if she killed both men, John would be a suspect, and she could no longer kill Magnussen without Sherlock witnessing, she shot Sherlock just below his chest where the wound would not surely kill him. She then hit Magnussen over the head and phoned an ambulance for Sherlock and left the building. When he was in the hospital, she told Sherlock not to tell John about any of it.
After Sherlock ran away from the hospital Mary found him after he fed her a series of clues. He projected a picture of her face onto an empty house – saying that it mimicked her personality, a façade. Inside the house, they discussed how she could easily have killed him yet chose not to. Mary said that she would do anything not to have John leave her. However, John was present while she confessed to being an assassin. They returned to Baker Street, where Sherlock told him as much as he knew about her. Mary produced a USB drive that she claimed had everything about who she used to be on it. She begged John not to read it in front of her, because she did not want to watch him stop loving her.
Six months later at Christmas, after a period of separation between Mary and John, John told her that he did not think her past was his business but that her future was his privilege. Mary, now known to be pregnant with a girl, was incredibly relieved. John admitted he was still angry at her, but the two remained together. After Sherlock kills Magnussen, Mary and John see him off in an emotional farewell when he is being exiled and sent on a dangerous mission as part of his punishment. However Sherlock ends up being flown back in when images of the deceased Jim Moriarty start appearing around London, indicating the late master criminal may still be alive.
Mary later gives birth to a healthy baby girl, named Rosamund Mary Watson, after her prior self and Sherlock, Molly, Mrs Hudson and possibly D.I. Greg Lestrade were named as Godparents. Alas, Mary is tracked down by a member of the assassin team she was part of and is forced to leave John and Rosamund behind whilst she takes random trips around the world, a new country, a new identity. Eventually, she is tracked down by Sherlock and John, but also Ajay, the fellow assassin she was running from. The three escape.
Mary meets Sherlock in an alcove to show him her past. She knocks him out using some sort of drug and leaves after he has seen the real Mary Watson, also known as Rosamund Watson. Her real maiden name is still unknown. We later find out, in an aquarium, that a government secretary, Vivian, gave the order in a fateful mission in Georgia (the country, not the state) that cost the lives of half of A.G.R.A., Mary's former assassin team. She then shoots Sherlock, but before the bullet could get to him, Mary intercepts it and dies from the bullet wound. John doesn't show up until she is shot, so he blames Sherlock for not protecting her and the two are not on good terms.
Mary Morstan appeared to be a sweet, kind-hearted and lively person. She is very intelligent, as she was able to tell when Sherlock was lying and keep the secret of her past from him. She cared a great deal about John and Sherlock.
Her relationship with John was threatened by the discovery that she was once an assassin. She has infrequently exhibited sociopathic tendencies, having a ruthless and violent side, as she was willing to murder Charles Augustus Magnussen in order to safeguard her secret from John.
She was cunning and calculating, a tactical adept. She was also a markswoman, an excellent shot with small handguns, showing reflexes and perfect timing, able to shoot a hole in a fifty-pence coin when tossed in the air.  She also had a photographic and extensive memory.  She is also able to mimic different accents, such as a New York one.
Mary Morstan and John Watson met while they were still under the impression that Sherlock was dead and while they had not known each other for a particularly long time, they loved each other enough for John to propose to her and Mary to accept. They eventually got married, and John also told Sherlock that both he and Mary were people that had changed his life completely and that they were the two he loved and cared about the most. Sherlock also figured out that Mary was pregnant with John's baby.  Their relationship remained stable until John heard Mary profess to having been an assassin, that she had been the one to shoot Sherlock and that everything he thought he knew about her past had been lies (something she hoped he would never find out). John got very angry with Mary and she was prepared for him to leave her, a thought she could not bear. However John eventually chose to accept that Mary had lied to him and not to have every detail about her past revealed to him, along with the fact that he must be attracted to danger as he had been drawn to people like Sherlock and Mary. John told her that the problems of her past were her business, but the problems of her future were his privilege. John threw the USB drive containing information on Mary's former identity into the fire, watching the flames consume it. Though he was still disappointed in her, they stayed together and loved each other. Mary was mentioned by Magnussen later as being John's "pressure point". 
Mary was initially shocked when Sherlock revealed he was alive but did not react as strongly as John did, and told John that she liked Sherlock when they were alone together.  Mary and John originally had to look after Sherlock, getting him out of the house and so on, showing that Mary cares for Sherlock, possibly as an "extension" of her love for John. She is fairly friendly towards Sherlock, considering his supposed psychopathic tendencies and the difficulties in making friendships these bring. He was the best man at her wedding to John and she even seemed to want all three of them to dance together. However Sherlock felt troubled when he realised Mary was pregnant, meaning she and John would hardly need Sherlock around with a real baby on the way.  Mary was considering coming to Sherlock as a client about her case with Magnussen. She is still not averse to hurting him, as shown when she shot him, though killing him would take it a step too far (showing a care not usually shown by assassins such as herself). She also purposefully shot Sherlock in a spot where it wouldn't surely kill him and called an ambulance right away, making sure he would be hospitalised and that his life could be saved. This made Sherlock defend her actions, though he admitted that she gave him mixed messages. Sherlock was less concerned about the fact that she was a former assassin than he was about her lying to John. But he reconciled with her just like John did and when he shot Magnussen, he exclaimed "Give my love to Mary", as this was something Mary wanted done and Sherlock had vowed to do anything for the couple, as he cared so much about John. He was pleased to hear Mary say that she would make sure to keep John in trouble when Sherlock was going on exile. 
Charles Augustus Magnussen
Magnussen heavily stalked Mary for her past life, and knew many personal details about her. This would drive Mary to attempt to murder Magnussen but she would be intervened by Sherlock Holmes. While going through his imaginary "files" on Mary in his Mind Palace, Magnussen referred to her as a naughty girl who had killed and harmed many people and claimed he understood why John liked her. Magnussen's knowledge of her previous life drove Mary to plan to murder him. Sherlock Holmes ended up doing this for her. 
- Mary is based on the character of Mary Watson née Morstan from the original Sherlock Holmes stories by Sir Arthur Conan Doyle.
- Her initials (R, part of the ancronym A.G.R.A) is a reference to the Agra Treasure, featured in The Sign of the Four.
- She shares traits with other Sherlock Holmes characters, such as Charles Augustus Milverton's unknown assassin in "The Adventure of Charles Augustus Milverton" (although unlike that version she doesn't accomplish killing Magnussen). Mary also shares skills with that of Professor Moriarty's henchman Sebastian Moran from "The Adventure of the Empty House".
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Moffat, Steven (writer) & Hurran, Nick (director). (12 January, 2014). "His Last Vow". Sherlock (2010). Series 3. Episode 3. BBC One.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Thompson, Steve; Moffat, Steven; Gatiss, Mark (writers) & McCarthy, Colm (director). (5 January, 2014). "The Sign of Three". Sherlock (2010). Series 3. Episode 2. BBC One.
- ↑ 3.0 3.1 Gatiss, Mark (writer) & Lovering, Jeremy (director). (1 January, 2014). "The Empty Hearse". Sherlock (2010). Series 3. Episode 1. BBC One.