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The police investigate the deaths of a series of people who all appear to have committed suicide by taking a poisonous pill.
They turn to their unofficial consultant, Sherlock Holmes, who deduces various elements pointing to a serial killer. Meanwhile, Sherlock is introduced to Dr John Watson by a mutual friend, and the pair immediately move into a flat at 221B Baker Street.
Dr John Watson, an ex-army doctor injured in the war in Afghanistan, is having a nightmare with images of armed conflict and injured people that make him wake up in sweat and tears. His therapist encourages him to start writing everything that happens to him in a personal blog as a means to cope with his stress symptoms and his trust issues; however, John replies that nothing happens to him.
There are three flashbacks into the previous year. On October 12th, a well-dressed businessman, Sir Jeffrey Patterson, is talking with his personal assistant on his mobile phone, asking for her help because he has gone to the wrong railway station. She tells him that he has no choice but to catch a cab, something that he is obviously not used to doing. It is also obvious that the two are having an affair. He is seen later on the floor of an office building nervously taking a capsule from a small glass jar and eating it, after which he has a seizure and dies. Later, the dead man's wife is seen giving a press release on how unexplained his suicide was while the PA cries on the background, trying not to be obvious.
The next is of November 26th; two young men walk in the rain under one umbrella. One, James Phillimore, tries unsuccessfully to hail a cab, then decides to go home to find another umbrella. His friend Gary Jenkins waits for a while and walks back to search for him, but the friend never comes back. James is seen crying in an empty gym while taking a capsule from a jar. We see a newspaper headline hit the front page, announcing the 18-year-old's suicide.
On January 27th, a wild party is in progress to celebrate the nomination of Beth Davenport, a local MP, to the Ministry of Transport. Two of the MP's assistants meet at the bar. One of them has removed the car keys from Beth's bag, in order to prevent her from driving They both suddenly look around, but their boss is not there any more. She is seen by her car, looking for her car keys, then looking around. She is then seen in a fenced yard full of rental containers crying, with a jar with capsules by her hand.
Later, the Metropolitan Police Service holds a press conference about the MP's death. Sergeant Sally Donovan confirms that the MP's suicide resembles those of Sir Jeffrey Patterson and James Phillimore, and hence all three incidents are being treated as linked. Detective Inspector Greg Lestrade notes that all three suicides happened in locations where the victims had no reason to be and they all took the same poison, but apart from that, there are no other links between the victims although the police are sure that they will find it. At this moment, every single mobile phone in the room receives a text message with the word "WRONG!". Lestrade ignores the messages, and states that they have their best people investigating the case, and again every phone is texted the word "WRONG!". When questioned about precautionary measures, Lestrade states that so far, there is no reason for anyone to take alarm, and again the text message is sent to all except Lestrade, who receives a different one "You know where to find me. SH". Once the conference is over, Donovan complains to Lestrade about the behaviour of the message sender, but Lestrade says that he does not even know how 'he' does it, so he is unable to stop it.
John is walking in the park when he is stopped by another man, Mike Stamford, who recognises him from their time as interns at St. Bartholomew's Hospital. They both sit to have a coffee and John's left hand spasm when talking about the army. John explains to his old friend that he might have to leave London, since he cannot afford an apartment on an army pension and it is very unlikely that he might find someone willing to share with him. Stanford tells him that he is the second person today that told him that same thing, and brings John to meet that person at the morgue.
In the morgue, we meet Sherlock Holmes, in the middle of performing an experiment by beating a corpse with a riding crop to prove an alibi and ignoring the clumsy efforts of the mousy assistant, Molly Hooper, to flirt with him. In the lab upstairs, Sherlock and John meet for the first time. Sherlock asks Mike Stanford for his phone because he needs to text and John offers his to him. He asks John if he had been in Afghanistan or Iraq, much to Stanford's amusement, then asks John if he would mind having a flatmate who plays the violin when he is thinking and goes on without speaking for days, to which John wonders how Sherlock knows that he is looking for a flatmate. Sherlock reveals that he knows that John is an army doctor invalided home from Afghanistan who has a brother of whom he disapproves because of his drinking habits, because he walked out on his wife, and that his therapist rightly thinks that his limp is psychosomatic. Then he gives his name, and his address, 221B Baker Street, before dashing off.
In another place, a woman with a pink skirt, jacket and shoes takes a small jar with the suicide pills from the floor, with a rather trembling hand.
John and Sherlock meet the next day to see the apartment. It is owned by a lady, Mrs Hudson, a widow who is giving Sherlock a special deal because he ensured the conviction and execution of her husband in Florida.
As they settle in, Sherlock eagerly asks for his opinion and is a bit hurt when John seems skeptical about Sherlock's claims, prompting him to retaliate and tell him that he could read John's military career from his face and leg and John's brother's drinking habits on the mobile phone. John asks how, but Sherlock does not answer. Mrs Hudson intervenes, asking Sherlock why he is not investigating the three serial suicides, just the type of case that would suit him.
Sherlock looks at the window and sees a police car with its emergency lights on parked outside, and deduces that there has been a fourth suicide and that something must be different this time. Lestrade enters the room running and answers Sherlock's questions with an address as he explains that this fourth victim is different in one way: she left a note behind. Sherlock asks about who is in charge of forensics, but when he hears the name of Anderson, he complains that Anderson would not work with him and that he needs an assistant. He tells Lestrade that he will go anyway. Lestrade and his men leave; Sherlock prepares to follow in a cab, but comes back and invites John along to view the crime scene. He knows John's seen in his army career enough blood and violent death for any man, but challenges him, "Want to see more?" to which John replies fervently, "Oh, God, yes".
As they ride the cab to the scene, Sherlock is texting on his phone while John stares. Sherlock finally tells John to ask the questions in his mind. The first ones are about where they are going and what does Sherlock do exactly, to which Sherlock replies that they are going to a crime scene where he will assist the police, as he is a consulting detective (the only one in the world: he invented the job). John replies that the police do not consult amateurs, and Sherlock retorts by explaining exactly how he deduced that John was an army doctor: the first was by his bearing and by the way he spoke. His injury occurred abroad, however judging from John's tan, which was only present on exposed skin, it was not a recreational visit. His limp is partly psychosomatic – something his therapist also believes – as John seems to forget he has it when standing still.
As for the brother, John's expensive mobile phone – a model only six months old – is inscribed with a message; "Harry Watson, from Clara XXX". It also had scratches and small damage around the charging port. From that, Sherlock deduces that the phone was a gift from a family member who is of a similar age, as no elder person would have such a new piece of technology. The original owner had a very heavy drinking problem, obvious to Sherlock due to the scratches around the plug-in port; the original owner's hands tremble when they plugged in the phone to charge each night. The fact that John was gifted with such an expensive present indicates that the relationship between Harry and Clara is or has failed, and Harry is the one leaving the relationship – if it were the other way around, the phone would be kept due to sentiment. They also want to keep in touch, however John does not go to him for help; thus, John must disapprove in some way.
Arriving at the crime scene, Sherlock asks if he got something wrong on his earlier deductions. John confirms the bad sibling relationship, the divorce and the drinking, but just one little error: "Harry" is short for Harriet – John's brother is in fact a sister. Sherlock acknowledges that he is not infallible. They are stopped at the yellow tape by Sergeant Donovan, who calls Sherlock a "freak", to which Sherlock retaliates by telling her that she did not make it home last night. John is introduced as a colleague, to Donovan's disbelief. Anderson, who is one of the forensic investigators, warns Sherlock that he does not want the crime scene contaminated, and Sherlock asks him if his wife is away for long. Anderson tries to dismiss it as a trick, but Sherlock points out that his deodorant is all over Sally and that the state of her knees indicates that she spent quite some time kneeling recently, leaving them both embarrassed while he and John enter the building. Once inside they are greeted by Lestrade who brings them to a room where the victim, identified as a woman by the name of Jennifer Wilson, lies facedown. Sherlock approaches the body and starts finding clues. He notices the message "Rache", scratched by the dying woman's fingernails. Anderson pipes up, mentioning that "Rache" is the German noun for revenge. Sherlock disregards Anderson, and completes the word into "Rachel", a lesson he learned from the Harry/Harriet error. He also notices that the dead woman's coat is wet, even on the inside of the collar, but the pocket umbrella is dry, suggesting that she had been in extremely windy, rainy weather – too windy for an umbrella.
Sherlock notices that the woman's jewelry is clean, save for her well-worn wedding ring. The inside is clean, however she had neglected the outside, indicating that she had been unhappily married for more than ten years; she does not bother to clean it, but regularly removes it to conduct more illicit sexual affairs with people she would prefer to believe she were unmarried.
Sherlock discovers, due to the weather, that she is from Cardiff and was in town for one night. He then asks John to examine the body, forcing Lestrade to acknowledge John as Sherlock's assistant, making the point that if the police want Sherlock's help, they have to do it his way. John tries to comply, but he is only able to say that the victim probably choked to death while seizing. Lestrade interrupts and asks Sherlock for his conclusions. Sherlock explains all that he has discovered, and asks where the case is. John cannot stop making comments on how brilliant he finds Sherlock's deductions, to the latter's surprised delight. Lestrade tells Sherlock that there was no suitcase, which triggers another set of deductions while Sherlock runs downstairs: the victim must have been accompanied by someone who took the suitcase, meaning that she was driven here and somehow forced to commit suicide by a serial killer, and indicating that the killer took the suitcase. It also means that the other three victims were all murdered.
John is left at the crime scene alone with the police and makes his way to the exit. Donovan warns him to stay away from Sherlock, because he is a psychopath and, one day, he is going to get bored and start committing crimes himself. As John limps towards a main road in search of a cab, a phone booth rings. The same happens to the phone in a shop as he passes by and then yet another booth elsewhere. The third time, he picks it up and a voice calls his name tells him to check three security cameras on the vicinity: All three move and focus away from the booth towards the other side of the street while a black sedan arrives, and John is instructed to enter it, which he does.
The woman in the car, who identifies herself as Anthea, but admits that it is not her real name, ignores his questions and keys on the mobile. He is driven to a warehouse where a tall man leaning on an umbrella awaits. He offers him a seat, but John defiantly remains standing. The mysterious man interrogates John about his relationship with Sherlock, to which John replies that he met Sherlock just yesterday, and then asks the man if he is Sherlock's friend. The man replies that Sherlock does not have friends, but he is the closest thing: an enemy, arch-enemy actually. John's mobile tells him that he has a text message – Sherlock, asking John to meet him at Baker Street whenever convenient. The man asks John if he plans to continue his association with Sherlock, to which John replies that it is none of his business. The man now tells John that he is concerned about Sherlock and offers John money in exchange for information on him, but John refuses while another text message rings, telling John that regardless of convenience, Sherlock wants to see him at their potential flat. The man tells John that he is very loyal very quickly to Sherlock and asks him if despite the trust issues described by his therapist, he has decided to put his trust in Sherlock. John turns back to leave, but the man sends a parting shot: his left hand (the one with the intermittent tremor) tells that he is going to move in with Sherlock, despite multiple warnings to stay away. Angrily, John turns around and the man asks him to show him his left hand. It is not trembling, even when the man touches it. The man tells John that when he walks with Sherlock, he sees the battlefield again, and that he should fire his therapist: John's hand does not tremble because he is stressed and haunted by war, or it would be trembling now since he is in a stressful situation, but because he misses it. John's phone signals a third text message: "Could be dangerous. SH" and John follows the woman back to the car, where he asks her to stop by his old apartment to take his gun, and then drive him to Baker Street. He also tries unsuccessfully to flirt with her. When John gets home, Sherlock asks him to text the message "What happened at Lauriston Gdns? I must have blacked out. 22 Northumberland St. Please Come," to Jennifer Wilson's phone number. Meanwhile, Sherlock retrieves a small pink suitcase from the kitchen and opens it, adding sarcastically to a surprised John that despite the suitcase and the text message, he is not the killer. Then Sherlock explains to John that he deduced that the killer drove the woman to Lauriston Gardens but made the mistake of driving away with the suitcase, a very colourful one (pink) which will draw attention especially on a man. He had to have gotten rid of it quickly, so Sherlock searched for potential dump sites near the crime scene located in areas accessible by car but isolated and found the right skip in less than one hour.
There is one thing missing: her mobile phone. A serial adulterer would be careful where she leaves her phone, so it is possible that the killer has it, on purpose or by mistake. The text message that John sent could only have meaning to the killer, so when John's phone rings, Sherlock realises that the hunt is on and asks John to come with him. John tells Sherlock about Donovan's warning, that Sherlock gets off on this, to which Sherlock replies that he said "dangerous", and John is here, proving that he is just as bad. Sherlock and John walk to a small Italian restaurant overlooking 22 Northumberland Street, where the owner, an old "friend" of Sherlock, insists on serving him and his "date" a nice free dinner. John tries unsuccessfully to explain that he is not Sherlock' date, but eats the dinner anyway while Sherlock ignores the food and watches the street. John profits for the moment by asking Sherlock some private questions. After an extraordinarily awkward conversation, in which it is established that John is straight and Sherlock "married to his work", Sherlock says he believes that the killer is someone who can stalk and approach victims on the streets of London without arousing any suspicion.
At that moment, a taxi stops in front of 22 Northumberland Street and the passenger in the back looks around but does not get out. Sherlock leaves the restaurant in a hurry, and John follows, his cane forgotten by the chair, at the same time as the taxi moves away. John memorizes the number plate but Sherlock visualises in his head the route (in an overhead grid view of the streets, showing traffic lights, one-way streets, and detours) that the taxi must follow to reach the closest main street and sees an alternative route via the alleyways and rooftops that will allow him to intercept it. He pursues it with John close to his heels but when they reach the taxi and meet the passenger, Sherlock realises that he is just an American tourist newly arrived from Los Angeles, judging from his teeth, his tan, and a tag on his luggage that has the initials "LAX" and "LHR"; meaning, the killer did not make it to the rendezvous point. Sherlock and John run back to Baker Street and, while still at the entrance hall, the doorbell rings. It is the restaurant owner, with John's cane. Sherlock texted him to have it sent back as a housewarming gift to John, since Sherlock had little doubt now that John would be staying with him in Baker Street.
Mrs Hudson greets them, panicking, because the police are upstairs and Sherlock and John go up to find their living room being systematically savaged by Lestrade and his people on a pretend drug bust. Lestrade wants to make a point too, plus recover the evidence from the suitcase that he was sure Sherlock would find. John laughs at the drugs excuse but Sherlock asks him to be quiet, indicating that Sherlock might have had some problems of that kind in the past. Anderson is all for arresting Sherlock as the murder suspect, but Lestrade wants Sherlock's insights. "Rachel", it has been discovered, was the name of the murdered woman's stillborn daughter, dead for fourteen years. Sherlock is dubious, and asks why the victim would have carved her daughter's name as she was dying, pondering outloud the probable connection to the case, she was obvious trying to pass on crucial information. Mrs Hudson enters and tells Sherlock that his taxi has arrived. Annoyed, he says he did not order one. Mrs Hudson is told about the drug search, and she complains about the mess. Sherlock explodes, shouting for everyone to shut up so he can think. The clues begin to gel for Sherlock. Mrs Hudson asks "What about your taxi?" Sherlock again responds in a rage, but stops short, pauses, and suddenly realises what he had been missing; he knew that the woman was clever, due to being able to hide a string of lovers, however he had failed to connect her intelligence with her death; she did not lose her phone – she planted the phone on her killer. As she did not have a laptop, the word she tried to scratch on the floor, "Rachel," was the password to her smartphone's account, which enabled those who knew it to activate the GPS and discover who had the phone.
Mrs Hudson again insistently mentions the taxi driver. Sherlock gets up from the laptop, and John sits down, discovering that that the phone is indicating it is at 221B Baker Street. Having just come up the stairs to the edge of the room, the cabbie, #71126, waits patiently. It dawns on Sherlock the cabbie fits the profile of the killer, and that he has the missing phone. While the others search, the cabbie takes the victim's pink phone out of his pocket and texts "COME WITH ME" to Sherlock's phone and returns to his cab. Seeing the message, Sherlock decides to follow cabbie outside without telling anyone. Downstairs, the cabbie confesses to the murders but warns that if Sherlock calls the police now, he will surrender quietly, but he will never reveal how his victims died. Sherlock will know the details only if he gets in the cab and goes with the cabbie. Sherlock enters the taxi while John, looking through the window, dials the victims phone. It rings unanswered in the cab. Lestrade and the police quit the apartment search, since the victim's phone is clearly ringing, but it is not ringing in the apartment.
The cabbie admits to Sherlock that he recognised him when he chased his cab. He had been warned about him and he read his website. Sherlock asks him who gave the warning, but gets no answer. The cabbie drives to an empty school building and briefly threatens Sherlock with a gun, but it is unnecessary since they both know that Sherlock will see it to the end. John, still standing next to the laptop logged in to the victims account, hears it beep as it updates the GPS location of the victim's phone, and he rushes off in pursuit.
Sherlock and the cabbie sit face-to-face in a large study room at the Roland Kerr Further Education College, where the cabbie challenges Sherlock to a supreme battle of wits, to the death. The cabbie puts on the table two bottles, each containing a large pill, explaining that one is quite harmless, but the other is deadly poison. When Sherlock selects the pill that he deduces to be safe, the cabbie promises that he will eat the other pill at the same time, and they will then find out who the "proper genius" is, and who is dead. Sherlock ensures that both of the bottles are identical, to which the cabbie verifies this fact. Sherlock has deduced the correct choice to make, and the cabbie asks Sherlock for his best game. The cabbie brags that he is not a novice, he has already won this game four times. He asserts that it is not a game of chance, but chess, confidently insisting for a second time that it is not a matter of chance, but genius.
John enters a building, searching it frantically and calling out for Sherlock, who is busy playing the cabbie's game. Summing up his observations, Sherlock ventures that the "game", as the cabbie calls it, is about his children.
The cabbie confirms he has been estranged from his children for three years and has been told that he is dying. He adds that the inheritance he will leave for them is only meagre, And in that moment, focusing on his children and his mortality, the cabbie looks to the pill bottle on his left. Sherlock deduces that is where death is.
The game over, Sherlock suggests to the cabbie that he could simply walk out, refusing the challenge, but the cabbie pulls out a pistol to force Sherlock to choose a pill. Sherlock chooses to be shot, having easily discerned the "pistol" to be a harmless cigarette lighter.
The cabbie pulls the trigger, and the lighter lights up. Sherlock gets up to leave, saying he looks forward to the court case, but the cabbie asks him if he figured it out. He taunts Sherlock to choose a pill again, to prove that he indeed has the superior wits to deduce the safe one, saying that Sherlock is driven by boredom and an addiction to thrill-seeking and is simply unable to resist the challenge. Sherlock explains that he solved the puzzle through "child's play".
Sherlock picks the bottle in front of the cabbie and they both face each other. John, searching frantically, sees Sherlock and the cabbie from a building across the street. With the cabbie edging Sherlock, both men prepare to take their pills. Sherlock delays, holding his pill up to the light inspecting it closely, when a crack shot is fired through a window pane, hitting the cabbie squarely in the chest. Sherlock runs and looks through the bullet hole in the broken window, to the building across the street, but he does not see the shooter. Sherlock returns to the cabbie and asks if he made the right choice, but the cabbie just smiles. Angry, Sherlock ruthlessly drives his foot into the cabbie's wounded shoulder, demanding the name of the serial killer's sponsor. In pain the cabbie screams "Moriarty!" and dies.
The incentive for the murders: Moriarty promised the cabbie to send money to support his children for each successful murder. These unsolved serial murders would attract Sherlock, the intended target. Moriarty presumed that Sherlock would not be clever enough to survive the battle of wits.
Outside, Scotland Yard has surrounded the perimeter and Sherlock is wearing a shock blanket, since he is being treated for shock. Lestrade questions Sherlock about the shooter and he starts to make some deductions. He suggests that the shooter was more than a crack shot, but also a skilled marksman who acclimatised to violence. However, the fact that they waited for Sherlock to be in immediate danger suggests that they have a strong moral code. Sherlock trails off as he looks over to John, realising that his new flatmate must have been the shooter. He turns to Lestrade and pretends to be ranting nonsense because of the shock and rejoins John outside the police lines. John pretends to have just arrived but Sherlock tells him that he knows the truth and expresses his concern for John, who has just killed a man. John admits that, but states that the driver was not a nice man, plus an awful cabbie. They both start giggling as they leave, and John asks Sherlock if he would have taken the pill. Sherlock tries to deny it, but John says that Sherlock gets his kicks risking his life to prove that he is clever, which just proves that he is an idiot like the rest of the human race.
At that moment, the tall man who "kidnapped" John appears and Sherlock recognises him. He wants Sherlock to join forces with him. A short squabble between the two reveals that the mysterious man is Sherlock's brother, Mycroft Holmes. Mycroft insists that he is a minor British government official, but Sherlock informs John that he is no such thing – he is the British government.
Sherlock and John walk away to a dim sum dinner while they talk about the new mystery man: Moriarty. Mycroft makes a note with Anthea to have Sherlock's and John's surveillance status upgraded to grade three active.
- The episode is loosely based on A Study in Scarlet.
- Sherlock's deduction based on John's mobile phone is drawn from an almost identical analysis of a pocket watch in The Sign of the Four.
- In A Study in Scarlet Sherlock uses a lost ring to lure the killer. In "A Study In Pink" Sherlock uses the mobile phone.
- The "three-patch problem" is a reference to the "three-pipe problem" in "The Adventure of the Red-Headed League".
- The mention by Mrs Hudson of "Mrs Turner who lives next door" is a reference to "A Scandal in Bohemia", where Sherlock's landlady is once named as Mrs Turner, instead of as Mrs Hudson.
- Sherlock's "The game, Mrs. Hudson, is on!" is a modernisation of the traditional screen Holmes' pronouncement that "The game is afoot", a line which appears in canon only in "The Adventure of the Abbey Grange", although the word "afoot" is used in numerous other phrases and contexts.
- John's reference in the final scene to having been shot in the shoulder (but developing a psychosomatic limp in the leg) is an allusion to a continuity error in the Conan Doyle stories: in the original A Study in Scarlet, John's injury is said to be in his shoulder, but in Conan Doyle's later Sherlock stories, it is said to be in his leg.
- The text messages Sherlock sends John ("Come at once if convenient", "If inconvenient, come anyway") are taken nearly verbatim from a telegram Sherlock sends John in "The Adventure of the Creeping Man".
- When John visits the flat in 221B Baker Street for the first time, Sherlock stabs a knife into a pile of papers. This is a reference to "The Adventure of the Musgrave Ritual", in which John mentions that Sherlock keeps "his unanswered correspondence transfixed by a jack-knife into the very centre of his wooden mantelpiece".
- James Phillimore, from "The Problem of Thor Bridge" died in the same manner, after going back for his umbrella.
- The whipping scene at St. Bart's in the beginning is a reference to a conversation between Stamford and Watson in A Study In Scarlet. Stamford tells Watson that Holmes "beats the subjects in the dissecting-room with a stick", "to verify how far bruises may be produced after death."
- The character Angelo (the "friend" of Sherlock at the restaurant) strongly resembles Joe Sisto. Joe Sisto appears in the 1946 film Dressed to Kill, starring Basil Rathbone as Sherlock Holmes. Sisto was accused of murder, but Sherlock was able the clear the charge by proving that Joe was busy at the time blowing open someone's safe. Joe Sisto felt grateful for clearing the charge and considers any friend of Sherlock Holmes a friend of his own.
- The address used to draw out the murderer of Jennifer Wilson - 22 Northumberland Street - was once the Northumberland Hotel, which featured in 'The Hound of the Baskervilles' when Sir Henry Baskerville came to stay in London before going out to Baskerville Hall.
- According to Molly's blog, it is January 29th when Sherlock comments on her lipstick.
- According to John's blog, the case is solved on January 30th.
- Sherlock messages from John's phone "If brother has green ladder arrest brother. SH". It is solution to "The Green Ladder" case, which is documented on Sherlock's website.
- In the park with Mike Stamford, John was holding a coffee cup showing the name of the coffee shop, Criterion. It is a reference to the Criterion Bar, where Watson and Stamford met in the canon.
- According to the episode commentary, Sherlock kissing Mrs Hudson on the cheek wasn't scripted. Benedict Cumberbatch had known Una Stubbs since he was very young, and it was his reaction on being able to work with someone who was like a mother to him.
- According to a news article that flashed briefly on screen in this episode, Roland Kerr Further Education College, where Jeff Hope died, was attended by James Phillimore, one of his victims.
- In the credits of Sherlock on Masterpiece Mystery on PBS, some of the letters were highlighted red. Letters from this episode spells out R-A-C-H-E-L which is what the Lady in Pink was trying to write on the floor.
- The episode was adapted into a manga by an artists named Jay. In 2016, it was published in English as a six-part comic book by Titan Comics with the help of the artist, titled Sherlock: A Study in Pink.
- When Sherlock and John get into the taxi in Baker Street (leaving to go to the scene of their first case) you can see a filming crane reflected in the back of the taxi.
- When the cabbie opens the bottle, he holds the pill in his left hand. but just before he is shot, the pill is in his right hand.
- When James Phillimore is in the gym, he is seen with three pills. Later on, when Sherlock and Jeff are in the study, there are only two pills.
- When Holmes and Watson walk out of the house after seeing it for the first time, a police car is visible over Sherlock's right shoulder, but when Watson walks past the police car vanishes.
- Donovan holds up the police tape for John after he asks her where can he get a cab, she is seen lifting it clearly above head height. In the next shot it's much lower.
- John's cane and his jacket shifts positions in the restaurant scene as the shot changes.
- The web page displayed for the man killed in his locked apartment displayed a file path in the address bar instead of a web address, revealing that it was just a document on a local hard drive.
- Sherlock Holmes – Benedict Cumberbatch
- Dr John Watson – Martin Freeman
- DI Greg Lestrade – Rupert Graves
- Mrs Hudson – Una Stubbs
- Molly Hooper – Louise Brealey
- Jeff Hope – Phil Davis
- Sgt. Sally Donovan – Vinette Robinson
- Ella Thompson – Tanya Moodie
- Helen – Siobhán Hewlett
- Sir Jeffrey Patterson – William Scott-Masson
- Margaret Patterson – Victoria Wicks
- Gary Jenkins – Sean Young
- Jimmy – James Duncan
- Political Aides
- Ruth Everett
- Syrus Lowe
- Beth Davenport – Katy Maw
- Mike Stamford – David Nellist
- Jennifer Wilson – Louise Breckon-Richards
- Anderson – Jonathan Aris
- Anthea – Lisa McAllister
- Angelo – Stanley Townsend
- Taxi Passenger – Peter Brooke
- Mycroft Holmes – Mark Gatiss
|Sherlock Holmes (Benedict Cumberbatch) • John Watson (Martin Freeman) • Mycroft Holmes (Mark Gatiss) • Jim Moriarty (Andrew Scott) • Greg Lestrade (Rupert Graves) • Molly Hooper (Louise Brealey) • Mrs Hudson (Una Stubbs)|
|Irene Adler • Mary Watson • Charles Augustus Magnussen • Eurus Holmes|
|"A Study in Pink" • "The Blind Banker" • "The Great Game"|
|"A Scandal in Belgravia" • "The Hounds of Baskerville" • "The Reichenbach Fall"|
|"The Empty Hearse" • "The Sign of Three" • "His Last Vow"|
|"The Six Thatchers" • "The Lying Detective" • "The Final Problem"|
|Unaired Pilot • "Many Happy Returns" • "The Abominable Bride"|
|"Sherlock: The Network" • Manga|