When Gregson's wife, Cheryl, becomes a victim of home invasion Sherlock and Joan work to find out who wants Gregson dead. When Sherlock discovers that the home invasion was just a case of mistaken identity, Gregson gets back on the case to track down the killer.
When two more bodies are discovered, the connection between is revealed to be military and stolen Afghan artifacts. They come across an excavator Beth, who claims her innocence and that her ex-husband couldn't possibly be the killer. When Sherlock and Watson discover the truth, they arrest Beth and her ex-husband for conspiracy and murder.
Watson is given a case to solve on her own, Sherlock ends up solving it himself. Joan is upset that he went behind her back to solve a case and to make up for it Sherlock gives her his old trunk with his cold cases in hopes that she will out smart him.
Gregson begins to reconcile with his wife.