From: ep. 3.1~3.3
Character: Mary Morstan
Reference: Anyone who knows anything at all about the Sherlock Holmes stories knows that in the very second Sherlock Holmes novel, The Sign of Four (only the second story that was ever published), Holmes’ client is a Miss Mary Morstan, with whom Watson proceeds to fall in love and to whom he becomes engaged at the end. In ep. 3.2 and 3.3 especially there are many little nods to Sign, not the least of which is ep. 3.2’s title: “The Sign of Three.” Character names like Major Sholto, and references in 3.3 to A.G.R.A. (the treasure that Miss Morstan inherited and Holmes and Watson investigated in Sign was called the Agra treasure), all point to this novel. Even Mary’s appearance is something of an echo of Watson’s description in the book (though in the show, she’s significantly more of a bad-ass):
Miss Morstan entered the room with a firm step and an outward composure of manner. She was a blonde young lady, small, dainty, well gloved, and dressed in the most perfect taste. There was, however, a plainness and simplicity about her costume which bore with it a suggestion of limited means. The dress was a sombre grayish beige, untrimmed and unbraided, and she wore a small turban of the same dull hue, relieved only by a suspicion of white feather in the side. Her face had neither regularity of feature nor beauty of complexion, but her expression was sweet and amiable, and her large blue eyes were singularly spiritual and sympathetic. In an experience of women which extends over many nations and three separate continents, I have never looked upon a face which gave a clearer promise of a refined and sensitive nature. I could not but observe that as she took the seat which Sherlock Holmes placed for her, her lip trembled, her hand quivered, and she showed every sign of intense inward agitation.
Sherlock’s best man speech also echoes Holmes’ reaction to Watson’s news of his engagement, at the very end of Sign, when he says “I cannot congratulate you.” Here is what Holmes says to Watson in the original:
He gave a most dismal groan. “I feared as much,” said he. “I really cannot congratulate you.”
I was a little hurt. “Have you any reason to be dissatisfied with my choice?” I asked.
“Not at all. 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. But love is an emotional thing, and whatever is emotional is opposed to that true cold reason which I place above all things. I should never marry myself, lest I bias my judgment.”
(all quotes are retrieved from the Project Gutenberg online edition of Sign of Four.)