The plot extends far beyond the opening events which briefly encapsulate an entire short story written by Sir Arthur Conan Doyle. In the 1904 story titled "Charles Augustus Milverton," Holmes and Watson break into a blackmailer's home to recover incriminating material, but must hide when Milverton returns. He is confronted by a masked shooter, who describes the devastation Milverton wrought in her life before she kills him. Holmes refuses to help find Milverton's well-justified murderer, and the short story ends as Holmes and Watson quietly discover her identity.
The wall hanging Joan gives Sherlock is the final stanza of the poem, "Stopping by Woods on a Snowy Evening" by Robert Frost. The poem is, in its entirety, as follows:
Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
When discussing the list of aliases used by Abraham Zelner there is an interesting demonstration of the differences between British English and American English; Holmes refers to the first letter of his name as "Zed" while Watson uses the American style "zee"