Sherlock investigates the death of a hotel manager whose body is found in a washing machine.
Tropes:
- Accidental Murder: Carly believes she killed her mother by instinctively shoving her when she grabbed her arm and causing her to fall and hit her head off her desk. She didn't, Geoffrey found Teri still alive and after he realized that Carly would make a better asset, he finished her off.
- Actor Allusion: Carly Purcell has a scholarship to the University of Michigan which Lucy Liu attended.
- Blackmail: Terri set up hidden cameras in the rooms of the hotel she managed and quietly encouraged call girls to work the bars and solicit the guests. It was assumed that she then used the footage of the resulting encounters as blackmail material against the various high-level businessmen and politicians who indulged this way. However, the actual purpose was merely to attract these men to her hotel so she could record the more valuable political and financial secrets they spoke about.
- Chekhov's Gun: The broken pen found on Mrs. Purcell's body. Sherlock notes it during the initial investigation but it's assumed it was empty. Near the end of the episode Sherlock sees a photograph of the pen and realizes it was filled with invisible ink that ended up on the real killer's shirt.
- Even the Girls Want Her: The call girl Holmes and Watson question about Terri Purcell takes a shine to Watson and hands over her business card, ostensibly for follow-ups but the way she focuses on Joan suggests otherwise.
- Exiled to the Couch: While in the Purcell home Sherlock notes the couch cushions are flattened in a way that indicates someone has been sleeping on them. Oliver Purcell claims that he started sleeping there due to insomnia but Sherlock counters that it doesn't explain why his toiletries are also in the guest bathroom. Indeed they turn out to be Russian spies whose marriage is a sham.
- The Handler: Geoffrey Silver, supposedly a family friend of the Purcells, was actually Oliver and Teri's handler.
- Mistaken for Cheating: A neighbor of the Purcells thinks that Teri Purcell has involved with Geoffrey Silver, not helped that Sherlock discovers that Oliver is unemployed and sleeping on the couch. The reason he is awfully close to the Purcell family is they are Russian spies and Silver is their handler.
- String Theory: Joan creates her own theory board for the Purcell case when Carly's "confession" doesn't sit right with her. The case is officially closed at this point but Joan's board draws Sherlock's attention to the seemingly-empty pen on Mrs. Purcell's body which turns out to be the vital clue to revealing the true killer.
- Undercover as Lovers: Oliver and Teri Purcell were Russian spies sent to pose as an American couple. Once Carly was born they ceased the physical part of their "relationship".
- Witness Protection: At the end of the episode Oliver and Carly are placed in Witness Protection for cooperating with the investigation. Oliver is also granted immunity for his previous espionage activities.
