A mysterious cipher is being scrawled on the walls around London. The first person to see the cipher is dead within hours of reading it. Sherlock plunges into a world of codes and symbols, consulting with London's best graffiti artists. He soon learns that the city is in the grip a gang of international smugglers, a secret society called the Black Lotus Tong.
- "Oh, I'm Sherlock Holmes and I always work alone because no one else can compete with my massive intellect!"
- ―John Watson, upset that Sherlock would not let him in.
At the National Antiquities Museum, Chinese pottery expert Soo Lin Yao demonstrates a tea ceremony to a group of visitors. Afterwards, as closing time approaches, her co-worker Andy Galbraith asks her out for a drink, but she turns him down and says that he would not like her very much if he knew her. Soo Lin then tells him to stop asking her out.
That night, Soo Lin is working late when she hears someone moving. She goes to investigate and finds a statue covered with a canvas. When she pulls the canvas away, she stares in shock at what she saw underneath.
Dr John Watson is trying to buy groceries at the supermarket, but the machine gives him problems and refuses to accept whatever form of payment he tries to use. Meanwhile, at home, Sherlock Holmes battles a robed swordsman and finally knocks him out. When John finally comes home, he complains that he had a row with a machine, and that Sherlock has not moved all day. Sherlock admits that he turned down a case about a diamond and gave the messenger a refusal. As he does, he also shoves a sword beneath the chair. John notices that Holmes is using his computer to check his e-mail. John asks for money, and Sherlock says he has to go to the bank, in actuality a big financial powerhouse.
At the bank, Sebastian Wilkes, an old university acquaintance of Sherlock's, asks for help. As they exchange greetings, Sherlock deduces from out-of-nowhere that Sebastian has made two trips around the world in the past month. He pretends to have learned this from Sebastian's secretary, but later confides to John that he figured it out from the fact that Sebastian's watch was brand new and it was also two days behind, indicating that he had crossed the International Date Line but had not altered it.
Sebastian explains that someone broke into their former chairman's empty office the night before and sprayed an apparently meaningless set of symbols on a wall and across the head of the chairman's portrait. Sebastian shows them the room, and two symbols, one on a painting and one on the wall nearby. The building is sealed and the CCTV shows that the vandal did the deed within a 60 second period between 11:33 PM and 11:34 PM. He offers to pay Sherlock to find the hole in their security. Sherlock turns down the money, but John quickly accepts.
Sherlock takes photos of the symbols in the room and then steps out on the balcony and considers the street far below. He then walks through the office, figuring out who has a line of sight to the chairman's office. He finds one office, which belongs to Eddie Van Coon of the Hong Kong division. Holmes notices that Van Coon's office has a direct line-of-sight view to the symbols. As he leaves with John, he explains that the graffiti was a message to someone, and Van Coon was the only one who could see the room where the message was left -since he trades with China, he probably works at night and the message was intended for someone who started their work at midnight.
Sherlock and John go to Van Coon's apartment. Sherlock buzzes Van Coon several times, but no one answers, but then he notices that the woman who lives directly above Van Coon has recently moved in – her label is still handwritten. Sherlock buzzes her, and pretends to be Van Coon. Claiming that he locked his keys into his flat, he asks if he can use her balcony. Sherlock jumps from her balcony onto Van Coon's directly below it, and lets himself in through the patio doors, which have been left unlocked. As he looks through the apartment, John buzzes the apartment to have Sherlock let him in. Sherlock forces open another set of doors and finds Van Coon, who has been shot in the head, a SIG-Sauer P226 pistol lying by his side.
As the police examine the scene, Sherlock determines from the laundry in Van Coon's suitcase that he had just come back from a three-day vacation, probably very recently. He also notices that something was tightly packed inside the suitcase at one point. The primary investigator, DI Dimmock, is prepared to regard it as a suicide, though Sherlock points out several holes in that theory – for one thing, someone appears to have inserted a black paper lotus flower into Van Coon's mouth post-mortem. Additionally, Van Coon was shot in the right side of his head, but he was left-handed (as evidenced by his habits and the layout of furniture in his apartment). He explains that Van Coon had been threatened (the spray-painted message from the bank is proof of this) and was waiting for the killer. He fired a shot at the killer when he came in, but the bullet missed and sailed out the open window. Sherlock tells Dimmock to check that the bullet is determined to not have come from Van Coon's gun.
That night, Sherlock and John encounter Sebastian while he is having dinner with several associates, and they inform him of Van Coon's death. He mentions that the police regard his death as a suicide, and berates them for being sidetracked.
Later that night, journalist Brian Lukis is seen running down a busy street, constantly looking over his shoulder, like he is being chased. He scrambles into his apartment, locks all of the doors behind him, and finds that his place has been searched. Panting, he turns, and his mouth drops at what he sees....
The next day, Andy is working at the museum when the director informs him that Soo Lin has resigned. Andy goes to her apartment in Chinatown and knocks on the door, but does not get an answer. He writes a note on museum stationary and places it in the mail slot.
John applies for the position of locum at a local clinic and admits to the doctor, Dr. Sarah Sawyer, that he needs the money. She warns that it is mundane, but he insists that that is fine and she hires him. He returns to 221B Baker Street and finds Sherlock contemplating the case, and notices that Sherlock has opened John's computer again, but not to check his email.
Sherlock has opened John's computer to a news website with an article about Brian Lukis, a freelance journalist who was shot and killed in his locked apartment by a killer who can apparently pass through walls. Sherlock notes that the circumstances of the journalist's death seem identical to Van Coon's death and suspects a connection. They go to Scotland Yard to make a request to see Lukis' apartment. While they are making their request to Dimmock, Dimmock confirms that the bullet extracted from Van Coon did not come from his gun. In Lukis' apartment, Sherlock finds a black paper lotus lying on the floor, identical to the one he extracted from Van Coon's mouth. Sherlock notices a skylight in the apartment, and realises that the two victims both thought that by locking themselves in, they would be impossible to reach.
He determines that the shooter broke into Lukis' apartment through the skylight, after climbing up the wall and running along the roof. This seems to indicate that the shooter is a human fly – he scaled a six floor apartment building to kill Van Coon, and ran along the roof and onto the terrace to leave the message at the bank. Sherlock finds a book recently checked out from a nearby library. Sherlock and John go to that library to look for the books that Lukis checked out, and find more of the painted graffiti symbols of the type from the bank on the shelf.
Back at 221B, Sherlock and John try to piece together the symbols. It is clear that the shooter threatened Van Coon by spray painting the message at the bank; Van Coon then went home, locked himself in, and hours later, Van Coons was killed. The shooter then left the message at the library for Lukis, and later that night, Lukis was killed. Sherlock realises that the graffiti symbols are ciphers of an ancient code.
He and John go to talk to an expert, a street graffiti artist named Raz, as he uses a spray paint can to paint his newest masterpiece on the door to a building. He informs them that he has exactly two minutes before two community police officers will come running around the corner and catch him in the act. Raz identifies the type of paint, but has no idea what the ciphers represent, and agrees to do some inquiries. There is no more time for conversation as two community officers come around and spot them. Raz and Sherlock run off, leaving John holding the bag with Raz's spray paint.
John returns home later, annoyed that he is been given an ASBO. To find out what links Van Coon and Lukis together, Sherlock traces Van Coon's footsteps on the day he was killed, and instructs John to track down addresses from Lukis' diary.
Sherlock talks to Van Coon's personal assistant, Amanda. She has all of his receipts, but notes that there is a gap in her boss' schedule on the day he was killed. Amanda admits that Van Coon was not appreciative of her – he only was appreciative of very expensive items – though Sherlock notices a bottle of expensive hand cream that he might have bought for her. Examining the receipts, Sherlock finds some very interesting details – Van Coon took a taxi out to the West End, but a ticket shows that he took the London Underground to get back, signifying that Van Coon lugged a heavy package up the escalator. There's also a receipt from a coffee shop that he stopped at on the way back.
Sherlock follows the receipts, and runs into John, who is retracing Lukis' steps. Sherlock starts to explain to John that Van Coon must have dropped off a heavy package in this area, and John immediately points to The Lucky Cat Emporium, an oriental curio shop across the street, noting that Lukis wrote down the address in his diary. Inside, they discover that many of the goods have numbers on them, in the same symbols as the ciphers. They realise that the numbers are in Hang Zhou numerals, an ancient Chinese numeral system now only used by traders – Sherlock determines that the numbers at the bank are 15 and 1. As they leave, a mysterious woman takes photos of them.
Sherlock and John go to a restaurant across the street for lunch, and Sherlock tells John about a possible motive for the killings. He remembers Seb telling them that Van Coon lost five million pounds one week, but made it all back the next. Now Sherlock knows why: Van Coon was smuggling illegal antiques. The idea that both Van Coon and Lukis were smugglers is not far fetched, given their cover careers: Van Coon was a businessman making frequent trips to Asia; Lukis was a journalist writing about China. Both of them must have smuggled things back in their suitcases, and used the Lucky Cat as their delivery point. Sherlock thinks that one of them stole something from his employer, but since the killer does not know which one took it, he had to kill both of them.
Sherlock is distracted when he notices a wet phonebook package next to an apartment door adjacent to the Lucky Cat, an apartment that just happens to be Soo Lin Yao's. He climbs up the fire escape and goes inside through an open window, ignoring John's pleas to open the door. Going over the apartment, Sherlock realises that someone else is inside and is still there. A shadowy figure ambushes Sherlock and chokes him almost to the point of unconsciousness, drops a black paper lotus on the floor, and just when Sherlock is about to pass out, the intruder suddenly releases him and runs out. Sherlock staggers downstairs and opens the door for John. He says they need to find Soo Lin. He also spots Andy's note, though he is noticeably wheezy from his near-death experience. John asks if he is getting a cold, but Sherlock insists that he is fine.
Sherlock and John talk to Andy, who says that Soo Lin resigned and the last thing he remembers seeing her do was demonstrate the tea ceremony. As Andy shows them her locker, Sherlock notices a nearby statue with two of the same ciphers on them, painted with exactly the same yellow paint. As the two men leave, the same woman from earlier watches them. Raz meets them and takes them to a tube station where someone has written more ciphers using the same paint. They spread out the search, and Sherlock finds a can of discarded spray paint. John finds a wall with more of the graffiti and snaps a photo. They go back home and John begins to pass out due to lack of sleep. Sherlock realises the numbers are all pairs – the killer left them to communicate with the thief and demand the return of the stolen item. They go back to the museum to ask Andy for more information. As they talk, Sherlock notices that one of the teapots Soo Lin was obsessed with has been freshly polished since the last time they were there.
That night, Sherlock and John discover Soo Lin in hiding at the museum, who explains that the code is the work of the criminal "Black Lotus Tong", of which she was once a member. She explains that the killer is Zhi Zhu, "The Spider". She shows them a Tong tattoo on the heel of her right foot, which resembles a lotus flower, and explains that everyone who smuggles for them bears this mark. She confides that the Black Lotus Tong employed her after her parents died and she had no other way to survive. By the time she was 17, she was smuggling thousands of pounds worth of drugs across the border into Hong Kong. She gave up the life and came to London, but Zhi Zhu tracked her down after five years. He asked for her help finding what was stolen, but she refused. When John asks how well she knows Zhi Zhu, Soo Lin explains that he is her brother. He became a puppet of the Tong in the power of General Shan, the Black Lotus' General. When she refused to help him, he accused her of betraying him, and left the ciphers behind as a threat.
Unfortunately, before she can decode the message, Zhi Zhu strikes again and fatally shoots her, leaving a lotus flower in her hand. He also goes after Sherlock and John, but despite firing at them, he fails to hit either of them.
After the incident, John and Sherlock talk to Dimmock at the police station. Based on what they learned from Soo Lin, Sherlock now believes that Van Coon and Lukis worked for the Tong. He also says he can prove it. At Barts Hospital, Sherlock flirts with Molly Hooper to convince her to bring the victims' bodies back out. As she is unzipping the body bags, he instructs her to only unzip the feet. Sherlock's suspicions are confirmed - both Lukis and Van Coon have the Tong tattoo on the heel of their right foot. To determine the identity of the item that was taken, Sherlock requests to have all of the books from both victims' apartments delivered to 221B Baker Street.
Sherlock and John go over the books, looking for the book that both of them had and has significant words that match the two numbers, page and word (the conclusion being that it refers to the first entry on page 15 of a book that both victims own). Dimmock returns the photo of the wall, and Sherlock explains that they thought Soo Lin Yao would decipher them. Performing a web search on recently discovered Chinese artifacts sold at auction, Sherlock finds the recent sale of two Ming vases that are being auctioned off by an anonymous source. He deduces that Van Coon and Lukis each brought back one of the vases when they returned on their latest trips to China – and it was their last delivery (Sherlock remembers having noted that something was tightly packed inside Van Coon's suitcase). Further searching for Chinese artifacts sold at auction shows in London over the past few months show that many of those sold came from anonymous sources. Even more startling is the timing: a cross-reference between the auctions and each victim's schedule shows that the auctions always coincided with Van Coon's and/or Luki's trips to China.
Sherlock is unable to find the proper book, and tells John they should go out to get some air. John tells him that he is going out on a date, and Sherlock gives him a pair of circus tickets in his name. However, when they arrive there, they discover Sherlock is there, and he admits that he bought a third ticket in his name. He explains to John that the Tong are using the circus as a cover to get the killer out of China – given that Chinese exit visas are hard to come by and those who do get them need very good reasons to leave the country – and he needs John's help to look around. John is more interested in getting off with Sarah.
They go inside and the circus matron first gives a demonstration of classic Chinese escapology, using a giant crossbow designed to fire at the escape artist when the weight of an emptying sandbag activates it. Sherlock sneaks off while the next act begins: a masked acrobat who "flies" on silk robes. Backstage, Sherlock spots the acrobat and realises it is Zhi Zhu. The matron comes backstage and Sherlock hides until she leaves. He finds a can of the spray paint and confirms it is the same type, and then the escape artist attacks him with a sword. The fight bursts onto the stage, and John and Sarah comes to Sherlock's aid. Sarah knocks him out with a stick and Sherlock confirms he has a Tong tattoo on his foot, of the same kind found on Van Coon, Lukis, and Soo Lin.
The trio returns to 221B and Sherlock figures that the Tong will have gone back to their hideout, the key to which rests within the ciphers. Sarah suggests that she should go and Sherlock agrees, but John hastily say she should stay and tries to find some food. Meanwhile, Sarah tries to make conversation with Sherlock without success. Mrs Hudson brings in some food, while Sarah notices that some of the ciphers are already solved – Soo Lin began translating two of the words and wrote them on the photograph they gave her. The two words are Nine Mill, and Sherlock realises it is the value of the stolen object (nine million pounds).
He leaves for the museum restoration room to find the book that she used, while at Baker Street, John suggests to Sarah that they get takeaway. The deliveryman arrives quickly and John answers the door. The man demands to know where the treasure is, and knocks John out when he says he has no idea what he is talking about.
Meanwhile, Sherlock tries to hail a taxi without success, and notices a pair of German tourists who are using an A-Z London guide. He remembers seeing it in a stack of books at Van Coon's apartment and in plain sight at Lukis' apartment, and deduces that both Van Coon and Lukis had the same book, as it is one that everyone would use, and the answer to the code lies in the first entry on page 15. He uses the tourists' copy to start translating the remaining ciphers, with this final result: Nine Mill for jade pin. Dragon den black tramway. He realises what has happened and rushes back to 221B and discovers that John and Sarah have vanished, and their abductors have left more ciphers. Sherlock goes over the maps of London to find the hideout.
John and Sarah wake up and find themselves in an abandoned tramway tunnel. The matron from the circus introduces herself as the Black Lotus General Shan, and mistakenly assumes that John is Sherlock. When he denies it, she points out that he has Sherlock's credit card, Sebastian's check to Sherlock, and the circus tickets in his name. Shan points out that they tried to kill him three times and failed, but they failed deliberately so that he would search for the treasure. Shan demands to know where the Empress' hairpin can be found. When John says he has no idea, they use the circus crossbow to threaten Sarah, and cut open the sandbag weight to activate the trigger, intending to kill her through a failed escapology performance. Sherlock arrives, knocks out one of her guards, and warns that her gunshot could easily ricochet. He kicks over the barrel to provide light and tries to untie Sarah, but Zhi Zhu strangles him with one of his silk ropes. John manages to knock himself over and jar the crossbow, which instead fires at and impales Zhi Zhu. Shan runs off and Sherlock frees Sarah, while John assures her that their next date would not be the same. When the police arrive, Sherlock lets Dimmock takes the credit, and Dimmock realises who he owes.
The next day, Sherlock and John go back to the bank, while Sherlock explains to John that when Van Coon and Lukis smuggled the Ming vases, Van Coon also stole a jade hairpin worth nine million pounds. John wonders how Sherlock is sure that Van Coon stole the hairpin and not Lukis, since Zhi Zhu did not know that.
As Seb hands John a check for finding the hole in their security (which is merely that they need to nail the windows shut), Sherlock talks to Amanda again. He reveals that she and Van Coon were having an affair – he remembers seeing an almost empty bottle of scented hand soap in Van Coon's bathroom, and hand soap would not be something Van Coon would keep unless he was having a woman coming over. Furthermore, the hand soap in question happens to be of the very same brand as the hand cream Amanda has on her desk. She admits to Sherlock that she ended the affair because Van Coon often took her for granted and they planned to go somewhere for the weekend, only he left on trips for China on short notice. Sherlock notes that Van Coon bought her a present to make up for his abandoning her, and asks for the hairpin that he gave her. When she hands it over, she mentions that Van Coon claimed to have purchased it at a street market, though Sherlock counters that he more likely pinched it. Van Coon never knew of its real value, but only the fact that it would suit her. When he tells Amanda the value of her hairpin, she runs off in shock.
A few days later, at 221B Baker Street, John and Sherlock read a newspaper story about Amanda's windfall ("Who Wants To Be A Million-Hair?" the headline reads), while Sherlock remarks on the irony that both Lukis and Van Coon died because Van Coon decided to pick up a trinket as a gift, with no idea of its real value. John realises that Sherlock is upset because General Shan escaped. John points out that they've cracked the code, but Sherlock says that the code can be easily changed – all Shan has to do is pick another book. As he looks out the window, John sees a Chinese street tagger writing another cipher on the wall opposite.
Back home, Shan talks to her benefactor, "M", on her computer, and thanks him for his help in getting them passage into London. She worries that his safety is compromised, but "M" assures her that her actions cannot be traced to him. When Shan promises that she would not betray him, the mysterious "M" has a sniper kill her.
Scenes cut from the final show
The following scenes were cut from the episode for time constraints, and are based on their appearance in the script:
- After the opening credits, and before the scene with John having a row with the checkout machine at the supermarket, a scene in which Van Coon jumps out of a taxicab upon getting home, darting into his building before the cabbie can ask him if he wants a receipt. He is seen rushing up the elevator, locks the chain bolt on the door, finds his SIG-Sauer P226 in a kitchen door, dashes into his bedroom, and jams a chair against the door.
- After Sherlock takes the jade hairpin from Van Coon's secretary, he and John return it to the museum, and the hairpin is added to the costume of the Empress Wu mannequin. He hands it to the museum director, while John hands Seb's check to Andy. A close-up shows Soo Lin Yao's name being added on to the list of museum benefactors.
- The concept of coded messages is taken from The Valley of Fear(using book references) and The Adventure of the Dancing Men (using pictorial messages). The markings on the feet of the Black Lotus members reference the markings of the "Scowrers" in Valley of Fear, along with the plot of escaping a secret society and being tracked and killed in England. Even the "a book that all would know" notion, comes from the same. The messages themselves, which appear to be plain graffiti, allude to the "Dancing Men", which appeared to be childish drawings, but are replacement ciphers known only by a criminal organisation.
- A conversation (SHERLOCK: This is my friend, John Watson. SEBASTIAN (latching on to the emphasised word): Friend? JOHN: Colleague.) mentions "my friend and colleague", by which Sherlock refers to Watson in The Adventure of the Noble Bachelor, A Scandal in Bohemia and The Man with the Twisted Lip.
- The scenes of Watson being kidnapped by criminals after mistaking him for Sherlock Holmes because he was mocking him was taken from Conan Doyle's short story The Man with the Twisted Lip.
- A murder victim found inside a locked room accessible only by climbing is an allusion to The Sign of the Four, as is the fact that the intruder had unusually small feet.
- According to a deleted scene, Sherlock later takes the Jade Pin to the museum where Soo Lin worked. John later uses the job's paycheck to make a donation in her name.
- According to Molly's blog, the date when Sherlock flirts with her to see the bodies in the morgue is March 25. Later that night, Jim begins targeting her.
- According to John's blog, the case is wrapped up by March 28 (it takes place between March 23 and 26). 
- "The Jaria Diamond case" which Sherlock tells Watson he didn't take with "Not interested. I sent them a message", is mentioned in John's blog..The rant with the checkout machines and getting a job at local clinic are also mentioned there.
- Jaria in "The Jaria Diamond case" is probably Jharia in Jharkhand state, India, which is well known for it's coal. It is supported by the fact that according to the script, the swordsman was wearing a full traditional battle dress and turban.
- In the scene where Sherlock checks a wall full of graffiti and posters, Production designer Arwel Wyn Jones graffitied his initials on it.
- Sherlock said John that his laptop password was "not exactly Fort Knox". Fort Knox is a United States Army post in Kentucky which houses a large portion of the gold reserves of the United States, and whose name is often a metaphor describing an impenetrable facility.
- Sherlock concludes "It’s highly unlikely that a left-handed man would shoot himself in the right side of his head. "Ironically John is left-handed but shoots with his right hand.
- The books common to both victims Sherlock compared are hard backed copies of Iain Banks’ “Transition” and Steven D. Levitt and Stephen J. Dubner's “Freakonomics”. For "A book that everybody would own." Sherlock looked in "the Concise Oxford English Dictionary", "Syphilis and local contagious disorders" by Berkeley Hill and "the Holy Bible".
- When Sherlock bumps into a couple of German tourist. A part of conversation is in German.
- (The German the tourists speak is not grammatically correct)
TOURIST: Hey, du! Siehst du nicht wo du hingehst? [Hey, you! Why don’t you look where you’re going?]
(Sherlock turns back and picks up the book, handing it back to the man.)
SHERLOCK: Entschuldigen Sie, bitte. [Forgive me, please.]
TOURIST (snarkily, snatching the book back): Ja, danke(!) [Yeah, thanks(!)]
(He turns away, putting his arm around his wife and still angry.)
TOURIST: Und dann sagen die, dass die Engländer höflich sind! [And they say the English are polite!]
(Sherlock turns and raises his arm to the cab again but it has already driven past. Looking up and down the road, he sees a Chinese couple, possibly father and daughter, standing at the corner over the road and consulting an A-Z as they too try to work out their route. Sherlock’s eyes narrow, he realises both Lukis and Van Coon had A-Z in their books. )
SHERLOCK (in flashback): A book that everybody would own.
(His memories move on to him smiling at Soo Lin after he handed her the teapot in the restoration room. On the table was a London A-Z.)
(In the present, Sherlock’s mouth opens in startled realisation and he breaks into a run, chasing back towards the German couple.)
SHERLOCK (shouting): Please, wait! Bitte! [Please!]
(The tourists turn back and frown in confusion as he hurries toward them.)
MALE TOURIST: Was wollt er? Was will er? [What did he want?What does he want?]
(Sherlock runs up to them and snatches the A-Z from the man’s hands and turns away, looking down at the book.)
TOURIST: Hey, du! Was macht du? [Hey, you! What are you doing?]
SHERLOCK (turning back to him momentarily): Minute! [Wait a minute!]
TOURIST (angrily): Gib mir doch mein Buch zurück! [Give me my book back!]
- In this episode, Seb Wilkes (played by Bertie Carvel) notes that he and Sherlock Holmes had been "in uni together". Benedict Cumberbatch and Carvel played college friends in the TV drama Hawking.
- Connie Prince is seen on the front page of The Sunday Telegraph Sherlock was reading on the end of the episode.
- In the credits of Sherlock on Masterpiece Mystery on PBS, some of the letters were highlighted red. Letters from this episode spells out I-M-A-G-I-N-E which was first word of page 15 of “Freakonomics” that Sherlock was looking through to decipher the code.
- It is never explained if Soo Lin ever met Van Coon or Lukis or at the very least knew that the Tong had London-based operatives.
- While Sherlock and Dr. Watson are viewing the webpage of the Crispian Antiquities website, the URL of the webpage is actually a path directory to a file on the computer's hard drive; the URL reads "C:\crispians\Crispian.html". The same goes for the Online News site (C:\http\onlinenews\online-news.html) and Quest Search (C:http\quest\questsearch.html) when Sherlock is looking for Chinese antiquities sold at auction.
- When Sherlock is backstage at the circus and finds the yellow spray paint, he draws a line on the mirror. In the next shot, the line he drew is completely different.
- When Sherlock and Watson first get to the bank, the time for New York is 7:45, then when Sherlock is trying to find out who the message is for, the time for New York is 7:20.
- In the bank where Sherlock is trying to get a view of the painting with the Chinese numbers, he backs into a pillar, which wobbles.
- At the end of the episode, Shan is sitting at a desk facing a wall without a window and yet the laser targeting ray appears directly on her forehead, which could only come through a window directly opposite her.
- Origami is Japanese, not Chinese.
- In the museum, Sherlock dives behind the large statue, which moves. That's unusual for a large statue, but looking closer,wheels attached to the base are visible.
- The number system referred as "Hangzhou" is actually Suzhou numeral system
- The graffiti in museum spells out 1-15,not 15-1.
- Third and forth pairs of numbers in the cipher were 36-37 and 70-95,but Sherlock reads them as "Thirty-seven, nine" and "Sixty, thirty-five".
- Sherlock Holmes – Benedict Cumberbatch
- Dr John Watson – Martin Freeman
- Mrs Hudson – Una Stubbs
- Molly Hooper – Louise Brealey
- Soo Lin Yao – Gemma Chan
- Sebastian Wilkes – Bertie Carvel
- Sarah Sawyer – Zoe Telford
- Andy Galbraith – Al Weaver
- Eddie Van Coon – Daniel Percival
- DI Dimmock – Paul Chequer
- Brian Lukis – Howard Coggins
- Museum Director – Janice Acquah
- Raz – Jack Bence
- Community Officer – John MacMillan
- Amanda – Olivia Poulet
- Shopkeeper – Jacqui Chan
- General Shan (credited as "Opera Singer") – Sarah Lam
- Surgery Recepionist – Gillian Elisa
- Box Office Manager – Stefan Pejic
- German Tourist – Philip Benjamin
|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|