The episode begins with a bald man watching a soccer game while assembling a gun. Behind him, on the floor, is a man, whose hands is tied and mouth taped. The man then arranges a tripod and then hooks up the tied man and hangs him upside down. The bald man then picks up a knife and slits the man’s throat. Next, Holmes is staring at a box full of bees and Watson wonders why the box inside the house is. Holmes tells her that he is planning to shift it into her room after she leaves. Watson wants him to carve out time to talk to discuss exit protocols. Just then Gregson calls. They arrive at the crime scene. The victim’s name is Vicars.
But there is no body; only a LARGE pool of blood! Holmes stares at the pool and deduces that there was a tripod device used to hang the victim upside down and slit his throat and let gravity work till the last drop of blood is drained. Watson wonders how he managed to deduce all this and Holmes tells her that he had stalked this mad man before in London. Holmes briefs the department about this murderer M. He has been active for the last 10 years and has tallied a body count of 37. He is clean in his methods and even his image has never been captured. Also, he has no “type” of victims and this makes it difficult to determine who will be next. He dumps the bodies in the ocean and as of now 21 have been recovered. He also kills in bunches and so Holmes warns the NYPD to be prepared for more bodies to be dropped.
Watson questions Holmes’ reaction towards this case and Holmes tells her that when he began with M’s first murder, he was an integral part of the investigation; but by the time the 36th murder took place, his addiction became out of control and he was useless to the Scotland Yard. Gregson feels that M has followed Holmes to NY and so he posts some uniforms in front of his house. Watson has an appointment with her therapist and Holmes asks her to leave immediately as he knows that he should get used to working without her. Next, M is in the hotel room with a prostitute and he pays her. She asks him if he would like one more go but he wants to see soccer. He then gets a text and he types in some code. He then brings out a briefcase and pulls out some letters cut out from a magazine to construct a note.
Holmes is examining Vicars’ body and Watson suggests that the body was dumped in a yard as there is high amount of engine oil on the hair. She then tells him that she is going to miss working with him. When they reach home, they find a note on the table from M. After reading the note, Gregson confirms that M has come to NY after Holmes and so he wants Holmes to move to a safe house. But Holmes denies and Watson says that she would stay with Holmes. Later, Watson sees that Holmes is looking at some real estate property. He says that he would move after they part ways. She goes to bed and he pulls out a book which has a hidden camera in it. He then watches M sneak into his house and stick the note on the lamp. Next we see that M is watching a woman walking her dog in the rain.
Just then a man walks up to him and tries to sell him some stuff. M is annoyed and asks him to back off. The woman enters her house and M walks off. Later, the guy is escorted by cops to Holmes’ house and he tells Watson that he is a pickpocket who Holmes uses at times to stalk people. He has come over to give information on M and collect his money. Watson is upset that Holmes enlisted some kids to stalk a dangerous man like M. Holmes reveals that M killed Irene Adler, the woman he loved. He says that he did not share information with the NYPD as he has no intention of capturing M; but he has every intention of torturing and murdering him. Watson is shocked. He then tells Watson that M murdered Irene. Before meeting Irene, he found women boring; but he was smitten by her.
Until her death, he used drugs recreationally; but after her death, he lost control and turned to opioids. He collects various tools that are hidden around the house and prepares to leave. Watson now realizes that Holmes kept her away from the M case and files, as he knows that she would see Irene’s name and figure out that he was up to something. he tells her that he has figured out where M is residing as he recognizes the scent on the note to be of a high end shampoo which is used in one of the hotels. She tells him that she will have to call Gregson and he tells her that she can do as she pleases. Holmes then goes to meet Teddy, his informant. Next, M is at the woman’s house he was keeping a watch on. He ties her up and sets up his device while watching soccer.
But Holmes walks up behind him and hits him hard with a baton. Watson goes to Gregson and tells him about the revenge Holmes is planning on. She tells him the name of the hotel M could be in. Now they need to look for both M and Holmes. Next, M wakes up and finds himself chained to a scaffold in a dark and isolated building. He then sees the torturing devices lined up and he asks Holmes how he is planning on using them. M then asks him why he did not hand him to the cops and he says it is because of Irene. M takes a while to remember that name and he then tells Holmes that it was not him. He says that he was doing jail time and he had read about Irene’s murder in the papers. He is disappointed in Holmes and the Scotland Yard for falling so easily for the copycat murderer.
Holmes is not convinced. At the hospital, the woman tells Gregson and Watson about Holmes who she refers to as the “second man” who freed her and asked her not to turn around. She remembers seeing white powder on his shoes. Watson knows where Holmes might be. She remembers that the property Holmes was looking at the other night was being renovated; it belonged to his father. In the building, Holmes realizes that M did not know that he was living in NY. M says that he is telling the truth. He says that he is an assassin and he has an employer. He did not kill Irene. He says that his name is Sebastian Moran and asks Holmes to look up on the web for his name. Holmes finds out that Moran is telling the truth and that he was serving time in prison during Irene’s death. Moran says he did not know that he was sent after Holmes.
He knows that his employer set him up and he is the one who killed Irene. His name is Moriarty. Looks like Moriarty set both of them up as piece of a puzzle. But Moran tells Holmes that Moriarty is the greatest puzzle he will ever come across. He tells Holmes that if he kills him now, he will be killing the best clue he ever had. He says that Holmes that he is an honorable man; but Holmes stabs him, proving him wrong. Gregson and the cops arrive at the building, but find it empty; only a pool of blood where M was hung from. Just then Bell calls Gregson telling him that Holmes walked into the station with M. M defends Holmes by telling them that he attacked Holmes and the stab wound was self defense. Watson finds Holmes sitting on the couch and she can tell from the stab wound that M is lying; looks like he was stationary and restrained. M is ready to confess about the murders and stick with Holmes.
Holmes tells Watson whatever M told him. Holmes tells her that he is going to miss working with her and leaves. Watson calls Mr. Holmes, Sherlock’s father and tells him that she is worried about his son and would like to stay on a little long. The next day she receives a message from Mr. Holmes saying that he does not want to continue her service and he will send her final check by the next day. But Watson lies to Holmes and tells him that his father has agreed to let her stay longer. Next, Holmes goes to the mantle and takes off all the clues that were pinned to the wall above it. he then puts up just one name; Moriarty. The episode ends.