A home intruder, who is after Gregson, is shot by his wife. Sherlock and Joan investigate who could be targeting Gregson. Two more victims, including Gregson's neighbor, are discovered murdered and Sherlock realizes the killer went to the wrong house and isn't after Gregson. Tying the victims to their service in Afghanistan guarding an archaeological site, Sherlock questions an archaeologist and quickly deduces that she was stealing from her digs and is now killing her accomplices with the help of her ex-husband. In the process of the investigation, Gregson must reveal that he and his wife separated. Meanwhile, Joan is frustrated with Sherlock after he solves a case she was asked to consult on. Sherlock gives her access to his box of unsolved cases.
