The beautiful and pure Mary Morstan is a single woman in her late 20s who comes to Sherlock Holmes for help in solving a mystery. She and Watson fall in love. Watson thinks Miss Morstan is quite lovely but fears money will make it impossible for him to marry her. He says of their prospects, "worse still, she was rich." He fears she will think he is pursuing her as a fortune-hunter—for her money, not herself.
As for Mary, she is impressed by Watson's stories of being sent to Afghanistan as an army doctor.
The two are middle-class English people, conventional, and not as intelligent as Holmes. They are well suited to each other. However, Watson does worry that marrying Mary will separate him from Holmes and his adventures. Watson says to Holmes,
Miss Morstan has done me the honour to accept me as a husband
This makes the decidely unromantic and analytic Holmes "groan."
Mary Morstan and Watson love each other, but Watson feels the tension between romantic love and male bonding.
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