** spoiler alert ** 
 Sherlock receives a letter addressed to Enola. They're both brought in on the case of a missing Duquessa, but Sherlock is stymied. He does find Enola, however, with the help of the family dog. When Sherlock, Mycroft and Enola search for and find the Duquessa, Mycroft realizes how erroneous his assumptions over Enola's needs are. They celebrate Enola's birthday together and decipher the message their mother has sent.