Sherlock's brother, Mycroft, arrives in New York and enlists Sherlock's help with a case involving his former fiancée, Nigella. Investigating the murder of Nigella's current lover outside of the stable where Nigella keeps her expensive stud. Searching the scene, Sherlock finds a set of four fingerprints leading to a cartel hitman, El Mecanico. Discovering Nigella's horse is an imposter, Sherlock saves Nigella's life, and investigates El Mecanico. After reviewing tapes, Sherlock zones in on a suspect with four fingers, but his prints don't match those found at the scene. They realize that El Mecanico had taken the prints off of his first victim – a homeless man he buried in the park years ago. Using Nigella as bait, NYPD captures El Mecanico, and Sherlock and Mycroft force Nigella to pay back victims of her scam. Meanwhile, Sherlock has difficulty accepting that Joan and Mycroft slept together while in London.