Sherlock's parents are played by actor Timothy Carlton and actress Wanda Ventham, Benedict Cumberbatch's real-life parents.
When John retrieves the tire iron from his car before entering the smack-house, there is a billboard behind him. The middle portion of the billboard has been torn down, revealing the old advertisement underneath. The combination of the new and old signs now read, "Information is power." This is the main plot of this story, as one man uses information to maintain power over the world via blackmail.
Mycroft, Sherlock and Watson each refer to "an east wind". This is a direct allusion to the closing part of Conan Doyle's story "His Last Bow" where Holmes says "There's an east wind coming all the same, such a wind as never blew on England yet".
Sherlock says he won the properties in Leinster Gardens from the Clarence House cannibal: "Quite a gambler, that woman." Until her death, Clarence House was the official residence of the Queen Mother, who was a very keen gambler.
The scene where John Watson enters a drug den is taken from the Sherlock Holmes story, "The Man With the Twisted Lip." In the original story, Dr. Watson goes to an opium den in the East End to retrieve a friend, Isa Whitney, and finds Sherlock Holmes among the opium addicts. In this episode, Watson is searching for a neighbor's son, Isaac Whitney, and finds Holmes in the drug den.