Amanda Abbington shared that the scene where Toby the hound wasn't moving was based in reality because the dog hired for the role wouldn't move for fear of the crowds. Moffat and Gattis then wrote this scene into the script.
Martin Freeman and Amanda Abbington were splitting up their sixteen-year relationship during the filming of this episode.
When Mary is in Norway, two boats can be seen with the names "Flekkete Bånd" and "Løvens Manke". These names both reference, and are direct translations of "the speckled band" and "the lion's mane", two well known tales of Sherlock Holmes.
Sherlock tells Mrs. Hudson to say the word "Norbury" to him if she ever thinks he is being overconfident or cocky. This is a reference to Doyle's short story "The Adventure of the Yellow Face" where Holmes tells Watson to do the same.
The deductions that Sherlock makes about the man with the large hands and the Japanese tattoo is similar to the deductions Sherlock Holmes makes in the original Arthur Conan Doyle story "The Red-Headed League".