All right, hear me out on this one. Look at a few elements re: her character in “The Empty Hearse”:
- She is indisputably in love with her significant other.
- One of Sherlock’s deductions about her reads simply “liar,” and we know that she did lie about liking the moustache.
- She is sent several threatening messages.
- Those messages are in code.
- She deciphers the code.
- Her significant other is in mortal danger at one point.
Is any of this sounding familiar?
Well, if you know your Canon, it might sound to you like the ill-fated Elsie Cubbitt of “The Dancing Men.” Elsie was an American woman with a mysterious past who fell in love with and married an Englishman. A man from her past, however—a criminal—threatened her in coded messages and ended up killing her husband.
Moffat and Gatiss create episodes and even characters that are composites of Canon elements and even Canon and pastiche. (The worst offender is “A Scandal in Belgravia,” which is far less “A Scandal in Bohemia” and far more The Private Life of Sherlock Holmes with a happy ending.)
Maybe, just maybe, Mary Morstan herself is a bit of a composite.