ट्रिविया

Sherlock and Mycroft's parents are played by Benedict Cumberbatch's real-life parents, Wanda Ventham and Timothy Carlton.

Two of the clients that come to visit Sherlock Holmes are a stepfather and stepdaughter. The stepdaughter is concerned about the sudden disappearance of her online admirer with whom she had regular contact. Sherlock deduces that the stepfather had been using an avatar on the internet to keep tabs on his stepdaughter's actions. This is a reference to Arthur Conan Doyle's short story 'A Case of Identity'.

The multiple alternate explanations for Holmes surviving the fall within this episode are, in part, based upon viable alternate fan theories discussed online.

Watson's fiancé, Mary Morstan, is played by Amanda Abbington, the then real-life partner of Martin Freeman.

The title of this episode reflects that of the first story in The Return of Sherlock Holmes, which tells of Holmes' first re-appearance after confronting Moriarty at the Reichenbach Falls and apparently dying, and was called "The Adventure of the Empty House".