Sherlock and Joan, back with the NYPD, investigate the murders of two men who looked practically identical. Meanwhile, Sherlock tries to get to the bottom of why his father is lingering in New York.
One of the murder victims, a German citizen, had changed his appearance slightly to resemble the other man. The team finds a link to a lookalike-finding software, which was created by a man named Dorian Moll, who says that a company, Countenance Technology, is trying to kill him, citing an attack the week before.
They talk to an executive, who claims that they had no reason to attack Moll, and that they were merely engaged in corporate espionage.
They eventually determine a link to an 11-year cold case involving a murdered fraternity pledge. One of the murder victims was trying to beat a DNA test by hiring a lookalike to take the test in his place. Unfortunately, someone took revenge for the original murder.
The dead pledge's brother readily confesses to the double homicide, but denies attacking Dorian Moll. Sherlock and Joan figure out that the Countenance executive was the one who did that as part of an attempt to cover up the fact that he had already beaten the same DNA test in the dead pledge case, and the executive is arrested.
Morland, after Sherlock assists him with a job, informs his son that he is sticking around in New York, much to Sherlock's disgust.