Dr. John H. Watson wakes up from a dream of his time in the war. He goes in to see a counselor, who warns that it will take a while for him to adjust to civilian life. She suggests that he write a blog about what happens to him, but John insists that nothing ever happens to him.
A lord is on the street and talking to his personal assistant by cell phone as he catches a cab. Later, he goes to an abandoned office building and takes a poison capsule. His wife, Www, makes a statement to the press and insists that her husband wouldn't have killed himself.
Two teenagers are walking in the rain. One of them goes to find a cab. Later, he goes to a sports center and kills himself with a poison capsule.
Beth Davenport, the Junior Minister for Transport, is celebrating her boss' nomination. She wanders off and goes to a rental container yard, where she kills herself with the same poison.
Later, Detective Inspector Lestrade holds a press conference and explains that there's no link between the three victims. All of the reporters get a text message saying that Lestrade is wrong. More messages of the same sort follow, each accompanying one of Lestrade's assertions. As the spokeswoman, Sgt. Sally Donovan, tries to close the conference, a reporter asks if they could be murders. Lestrade tries to reassure everyone. Another text message comes in, telling Lestrade he knows where to find the texter, SH. Afterward, Donovan tells Lestrade to get the caller to stop, but Lestrade admits he doesn't know how the caller does it.
As John walks in the park, he meets an old army friend, Mike Stamford. They talk and John admits he's not the man that Mike once knew. Mike suggests that John get a roommate, and suggests that he met someone else earlier who needed a roommate.
At the morgue, Sherlock is examining a corpse. He tests a riding crop found with the victim to determine when the bruises appear. Sherlock notices that the coroner, Molly, refreshed her lipstick, and she suggests they get coffee. He asks her to take his order. As he works at the lab, Mike brings John in. Sherlock asks for him phone, saying he prefers to text, and John loans him his phone. Once he has the phone, Sherlock immediately deduces that he's a soldier. He asks John how he feels about the violin, and explains that he plays it when he's thinking. Sherlock then deduces that Mike brought John in to introduce him as a roommate, and tells him to meet to look at a flat the next day. John, a bit overwhelmed, admits that it's somewhat sudden. In response, Sherlock immediately announces any number of things about John that are 100% correct, and leaves.
A woman considers killing herself with the same poison capsules.
Sherlock and John meet at 221B Baker Street, and he explains that the landlady, Mrs. Hudson owes him a favor. They look around and agree to move in together. John notes that he looked up Sherlock on the Internet and found his website. Mrs. Hudson mentions the suicides, and he tells her that there's been a fourth and there's something different. Lestrade arrives and explains that the newest victim left a note. Sherlock refuses to drive in the same car, and warns that he needs an assistant. Once Lestrade leaves, Sherlock laughs in triumph, tells John not to wait up, and leaves. However, he returns a minute later and asks John if he saw a lot of injuries as an army doctor. Sherlock invites him along and John eagerly agrees.
As they take a taxi, John wonders who Sherlock is. Sherlock explains that he's a consulting detective and how he deduced everything he announced about John. They arrive at the crime scene and John admits that Sherlock was right. The PR woman, Sally Donovan, greets them and isn't impressed with Sherlock's presence. The coroner, Anderson, comes over, and complains that Sherlock is there. Sherlock concludes that Sally and Anderson are having an affair.
Sherlock and John examine the crime scene. The dead woman, Jennifer Wilson, is wearing a bright pink dress, and scratched the letters RACHE n the floorboards. Sherlock examines the body and concludes that she was unhappily married and a serial adulterer. Anderson figures that she's German, but Sherlock dismisses his conclusion and invites John to examine the body. As Lestrade comes in, John suggests that the victim died of asphyxiation. Sherlock notes that she must have a suitcase based on the splash patterns on her leg, and wonders where it is. When Lestrade figures she was writing German, Sherlock tells him that she was writing the word Rachel. Sherlock tells Lestrade that they're serial killings, not suicides, and demands to know where her case is. He realizes that the killer drove her there but forgot to take the suitcase out.
Outside, John looks to hail a cab, and Sally wonders who he is. She notes that Sherlock doesn't have any friends, and warns John to stay away. When John wonders why, Sally explains that Sherlock gets off on crime, and the weirder it is, the better. She figures that he's a psychopath, and eventually he'll start create murders of his own.
As John limps down the street, a phone in a nearby phone booth rings. He ignores it and continues, but another public phone rings at a café as he walks by. At the next phone booth, the phone rings and John picks up. The man at the other end says there are three security cameras nearby, and demonstrates that he controls them. A black sedan pulls up and the man tells John to get in. He does so and finds a woman sitting there. She advises him that there's no point in asking where he's going.
The sedan takes John to a warehouse. The caller admits that he didn't call because he didn't want to draw Sherlock's attention. He wonders what Watson's involvement with Sherlock is, and says that he's what Sherlock would consider an archenemy. John gets a text message from Sherlock saying to meet him back at 221B. The man tells John that he'd be happy to pay him money in return for information on what Sherlock is up to. He admits that he worries about Sherlock, but wants his concern to be unmentioned. John refuses and walks away, and the man notes that John won't listen based on his left hand. He points out that John has an intermittent trauma, but he doesn't have it right now. The man concludes that John misses the war, and welcomes him back. As he walks away, he tells John that it's time to choose a side.
The woman takes John back to 221B. As he goes, he asks her out on a date and she casually dismisses him. Inside, he finds Sherlock using three nicotine patches to help him concentrate. He asks for John's cell phone so he can use it to send a text number, because his phone number is on his web site. Sherlock says that the killer made the mistake of taking the suitcase by accident, and asks John to send a text to the phone number on his desk. John tells him that he met Sherlock's archenemy, and Sherlock wonders if he offered John the money. He explains that the man is the most dangerous man he'll ever meet, but they need to focus on the current case. Sherlock then has him call Jennifer Wilson's number and send a text message: "What happened at Lauriston Garden? I must have blacked out. 22 Northumberland Street, please come."
As John does so, he realizes that Sherlock has Jennifer's case. Sherlock explains that he knew the murderer would have gotten rid of it within five minutes, and checked every alleyway that would fit the car. There's no phone in the case, so Sherlock had John text her phone, believing the murderer has it. Sherlock's phone rings, and Sherlock explains that the murderer would panic after receiving such a call, but anyone else would ignore it. Sherlock prepares to leave, and invites John to come along. John tells him what Sally said, but Sherlock points out that he came along anyway.
Sherlock and John go to 22 Northumberland Street and set up at a restaurant across the street. The owner, Angelo, explains that Sherlock cleared his name of murder. As they wait, John insists that people in real life don't have arch enemies. Sherlock wonders what real people have, and John says that most people have friends. Sherlock finds it dull, and insists he doesn't have any girlfriends or boyfriends. He assumes that John is interested, and John insists he isn't.
A taxi pulls up and Sherlock concludes that it's the murderer. The driver notices Sherlock watching and drives away, and Sherlock computes where he'll be given traffic routes and closures. They run off, John leaving his cane at the restaurant. They ascend to the rooftops and jump across, and then come down again. They catch up to the cab, but discover that the passenger is recently arrived from London and thus can't be the murderer. They quickly run off as the passenger reports the encounter to the police.
John and Sherlock return to their flat, and Sherlock tells Mrs. Hudson that John will be taking the room. There's a knock at the door and John finds Angelo there with John's cane. They go upstairs and discover that Lestrade is conducting a drug bust, and brought all of the officers who dislike Sherlock. Lestrade explains that he's doing it to intimidate Sherlock into cooperating with him. They've found the case and note that Sherlock said the murderer would have it. Lestrade explains that Rachel was the name of Jennifer's stillborn daughter, but Sherlock doesn't buy it.
Mrs. Hudson comes in to tell Sherlock that there's a taxi out front. He snaps at her, and then realizes that Jennifer planted her cell phone on her murderer, so it would lead them to her killer via its GPS tracker. There's an e-mail address on the luggage, and Sherlock realizes that her smart phone's password is her dead daughter, Rachel. He uses the computer to track tracks the GPS signal to 221B Baker Street. They start looking for a mobile phone, and Sherlock realizes that the taxi driver has it, and he collected each of the victims.
The driver, standing at the door, sends a text to Sherlock to come with him. Sherlock goes out for what he claims is some fresh air and gets in the taxi. He identifies the cabbie as the one who stopped at Northumberland, and the cabbie admits that no one ever remembers cabbies. He tells Sherlock that he'll confess to the police if Sherlock calls them, but he knows Sherlock won't do it. The cabbie tells Sherlock that he never killed anyone, but talked to the victims and then they killed themselves. When Sherlock notes that at least he'll stop the killings, the cabbie tells him that's not the result he wants. All he wants to do is give Sherlock a ride, have a talk... and then Sherlock will kill himself. Sherlock hesitates for a moment and then gets into the cab.
Upstairs, John notices that Sherlock has driven away in a cab. He tries to call Sherlock's phone but gets no answer. Sally insists that Sherlock is a lunatic and wasting their time, and Lestrade finally agrees.
The cabbie explains that he was warned about Sherlock, but will only say that the person who warned him is someone who has noticed Sherlock. He refuses to say anything further.
Lestrade wonders why Sherlock would have left, and admits he only puts up with him because he's desperate.
The cabbie takes Sherlock to an educational college and draws a gun on him, inviting him to go in. However, he then puts it away and walks inside, confident that Sherlock will follow him.
Once the police leave, John realizes the cell phone GPS signal has moved, and leaves.
The cabbie sits down at a table and invites Sherlock to join him. He takes out two bottles, each one with a pill, and says that he's a genius as well. The cabbie then explains that one pill is poison, and the other one is safe. Sherlock will have to choose, and he wonders why he should. The cabbie says he'll take the pill from the other one at the same time, and he did the same thing with his other four victims. When Sherlock notes that it's chess, the cabbie observes that he survived four times and it's Sherlock's move. He pushes one bottle toward him and lets him chose.
En route, John tries to call Lestrade.
Sherlock refuses to play, insisting it's luck, but the cabbie says that he knows how people think and that they're all stupid.
John follows the signal to the college.
Sherlock wonders why the cabbie risked his life four times, and deduces that the cabbie is dying of an aneurysm. Despite the fact he's estranged from his family, he still loves them. The cabbie admits that someone is paying for his family's support in return for his playing the game. Sherlock wonders what happens if he walks away, and the cabbie draws a gun. Unimpressed, Sherlock asks for the gun, and the cabbie pulls the trigger, revealing it's a novelty cigarette lighter. As Sherlock leaves, the cabbie asks if he figured out which one is the safe bottle. Intrigued, Sherlock returns to the table and takes a bottle. The cabbie takes the pill out of his bottle and invites Sherlock to give it a try.
John bursts in to the room across the way and sees the two men. Meanwhile, the cabbie figures that Sherlock gets bored so easily, and that he needs to prove that he's clever by taking the pill. Sherlock takes out his pill and considers it. As he starts to put it in his mouth, a shot rings out. John has shot the cabbie from across the way and then departed. Sherlock demands to know if he was right, but the dying cabbie doesn't say anything. He then asks who his fan was, and warns that there's still time to hurt him. When the cabbie refuses, Sherlock steps on his wound until he screams a name: Moriarty.
Afterward, Lestrade tells Sherlock that they couldn't find the shooter. Sherlock deduces that the shooter was someone with military service and nerves of steel... and notices John standing nearby. He tells Lestrade to forget everything he just said. Sherlock then goes over to thank John, and wonders if John is okay with killing a man. John concedes that the cabbie wasn't a nice man. As they walk away, Sherlock claims that he wasn’t going to take the pill. John doesn't believe him.
As they walk away, the man from the warehouse drives up. Sherlock knows who he is. The man congratulates Sherlock and admits that he's concerned about him. When he points out that their feud always upset their mother, Sherlock addresses him as Mycroft and introduces him to John as his brother. As Sherlock goes, Mycroft insists that he's truly concerned. John goes after Sherlock, who admits that he's intrigued by Moriarty.
Mycroft and his secretary consider John and Sherlock. He wonders if John could make his brother better... or worse, and tells his secretary to upgrade their surveillance status.