"Fourth floor. That’s why they think they’re safe. Put the chain on the door, bolt it shut. They think they’re impregnable. They never consider for a moment there's another way in."Holmes is hired by an old frie-... acquaintance to investigate a mysterious break-in at a bank in the City. Although nothing was stolen, the bank are concerned that someone was able to get in undetected and want Sherlock to find out how, but Sherlock is more interested in the symbols the intruder spray-painted onto an office wall. He discovers they are a coded message intended for an employee of the bank, who is later discovered dead in his flat. The next day, a journalist is killed and the same symbols are found nearby. Holmes and Watson follow a trail of clues that link the two dead men to a Chinese smuggling ring, who are trying to retrieve a valuable item that one of them stole. Holmes eventually cracks the coded message based on Suzhou numerals and a book cipher, but not before Watson and a female friend are kidnapped by the criminals. Holmes rescues Watson's friend but the leader of the gang escapes. After escaping, the leader of the gang is in communication with her superior, who is identified by the initial "M". She is then shot by a sniper. Holmes figures out that the banker was killed because he took a piece of jewellery from one of his shipments as a gift to his girlfriend, not realizing how valuable it was.
This episode provides examples of the following tropes:
- Absentee Actor: Mycroft and Lestrade.
- The Book Cipher: Sherlock and John encounter a number of symbols. This turns out to be numbers written in an ancient Chinese script, with the book being a Tour Guide of London (which ends up as part of Fridge Brilliance, as the Chinese Gang use these symbols to arrange meeting points.
- Bound and Gagged: Sarah is bound, gagged, and facing a giant crossbow.
- Chekhov's Gun: Quite a few. John ends up holding onto Sherlock's debit card, a cheque for Sherlock from the bank that hired him, and tickets reserved under the name Holmes; since John is holding them all by the conclusion, they all result in John being mistaken for Sherlock.
- Disposable Love Interest: Sarah for John, they seem to be getting on great and she doesn't even hold it against him when his sideline almost gets her killed by Chinese gangsters, plus she takes an interest in the case and helps save Sherlock from one of the thugs. But then she and John split up offscreen for reasons unknown and she isn't seen again.
- Dramatic Drop: Soo Yin would have broken one of the teapots she cared for if not for Sherlock catching it — although he was the one who startled her into dropping it in the first place.
- Explain, Explain... Oh, Crap!: Sherlock is looking through a flat and talking to John (well, sort of- John's left grumbling outside the door, can't hear a thing and might as well not be there).Sherlock: Someone else has been here. Somebody else broke into the flat and knocked over the vase, just like I did.
John: You think maybe you could let me in this time? Can you not keep doing this, please?Sherlock: I'm not the first.
John: What?Sherlock: Somebody's been in here before me.
John: What are you saying?
- Hidden in Plain Sight: Accidentally. The treasure that's worth nine million pounds is being used as a hairpin.
- I Can Explain: John gets an ASBO after being mistaken for a graffiti artist.
- Idiot Ball: The Blind Banker more or less revolves around the villains being complete morons at every turn. Killing two operatives who they could have tortured to get info out of — just to get Sherlock's attention? Mistaking John for Sherlock? Only hiring Moriarty to get them into the country instead of to find the pin? You almost can't blame Moriarty at the end; Shan was clearly Too Dumb to Live.
- Fridge Brilliance: They were commanded by Moriarty, who wouldn't care if they got the pin. He just wanted to screw with Sherlock.
- John, a trained soldier, leaving a defenceless woman alone to run after Sherlock, while knowing that someone is out to kill her.
- Sarah sitting calmly watching the iron ball (which will trigger the crossbow) descends... instead of just rocking the chair left-right until she falls down out of the way or the bolt (none of the bad guys is stopping her or even paying attention to her at that moment). And she is a doctor?! Double facepalm.
- Imminent Danger Clue: The open window, signalling the presence of the acrobat assassin.
- It Was Here, I Swear!: In the second episode, John discovers a wall covered with vitally important graffiti; in the ten minutes it takes him to fetch Sherlock to show him the evidence, however, it's been painted over. Subverted; Good thing John took a picture with his camera phone.
- The Killer Was Left-Handed: Sherlock concludes that an apparent suicide was in fact a murder, because the left handed victim had been shot on the right side of his head, something which would be rather awkward to achieve with your left hand.
- Le Parkour: Zhi Zhu is a parkour expert.
- Look Behind You: Used by Sherlock in the first five minutes, against a sword-wielding assailant. Surprisingly, it works.
- Meaningful Echo: Also added to through speech when John sarcastically imitates Sherlock; by shouting "I'm Sherlock Holmes, and I always work alone because of my massive intellect!", it was assumed by the assassins that he was actually Sherlock and just boasting about it.
- Mistaken Identity: The crime syndicate believes John to be Sherlock because of a series of unfortunate coincidences.
- Mythology Gag:
- The plot draws on two Sherlock Holmes stories, The Sign of the Four (oriental treasure, foreign killer who can climb into apparently-inaccessible places, Watson finds romance) and "The Adventure of the Dancing Men" (mysterious symbols left in a public place as a message for woman trying to leave behind her dark past).
- The sequence with Sherlock and John trying to figure out which book is the basis of the book cipher takes its cues from a scene in The Valley of Fear.
- Poor Communication Kills: The Tong. All they had to do was explain to their smuggler that the pin he took was actually worth 9 million. He had no idea of its value and gave it to his girlfriend as an apology for breaking their date.
- Redemption Equals Death: Soo Lin.
- The Reveal: The spray-painted symbols are ancient Chinese numeric symbols—each referring to a page in a London street atlas that leads to the smugglers' hideout.
- Sarcasm-Blind: The acrobat stops strangling Sherlock after John imitates him, leading the Acrobat to think that John is Sherlock.
- Screw This, I'm Outta Here!: Sherlock and the graffiti artist both scarper when the police show up, leaving John holding the bag.
- Speak Friend and Enter: John finds a wall covered in graffiti that's actually the code that will allow them to solve the case. He runs to fetch Sherlock, but by the time they get back, the entire thing's been painted over. Sherlock then spends the next minute or so pressing John to remember the code, waxing lyrical about the fallibilities of the average human mind's memory capacities, all the while ignoring John's protests and attempts to get to his camera phone, on which is stored a picture of the entire wall before it was painted over.
- Staggered Zoom: Onto the face of the second murder victim.
- The Triads and the Tongs: A group of Tong serves as the main villain, though it's referred to as an ancient Chinese crime syndicate rather than a Chinese-American organization a little over a century old ( and, as with the previous episode's villain, it turns out that Moriarty's The Man Behind The Dragon Lady).
- They Wasted a Perfectly Good Sandwich: John orders a meal at a Chinese restaurant while he and Sherlock ponder their situation. John only gets two bites before Sherlock drags him away.
- Uncovering Relationship Status: Sarah mentions John's girlfriend, he replies that he doesn't have a girlfriend and invites her on a date.
- Wham Line: When Sherlock is following the trail of the killer, he starts inspecting the apartment of the missing Soo Lin:Sherlock: Why didn't he close the window when he left? [realisation] Oh. Stupid, stupid. Obvious: He's still here.
- Wrong Genre Savvy: The Tong seems to completely believe the reason why the item is missing is because one of their smugglers stole it for its value and set out to kill them as punishment. It turns out the guy just got it as an apology gift for his girlfriend and had absolutely no idea of its value.
- Yellow Peril: Several reviewers have noted that the portrayal of the Chinese villains (and a random Arabian Nights Days assailant wielding a freaking scimitar) smacks uncomfortably of this trope.
- A particularly interesting change considering that, while Yellow Peril villains were common enough when the original "Adventure of the Dancing Men" was written, the villains in that story were actually Chicago gangsters with a secret code.
- You Have Failed Me: In this respect, "M" is a traditionalist.