At a museum, Soo Lin Yao demonstrates a tea ceremony and how tea pots are aged. Another attendant, Andy Gilbraith comes over and talks to her, and Soo Lin notes that the pots are cracking. Andy asks her out for a drink, but Soo Lin says he wouldn't like her that much. She refuses Andy's invitation and tells him to stop asking.
The museum closes for the night and Soo Lin works late closing up. She hears someone moving in the museum and goes to investigate, and finds something shocking on the statue beneath a cloth.
While John gets groceries and tries to deal with a variety of problems, Sherlock is at the flat dealing with a sword-wielding robe-wearing attacker. He finally knocks the man out and goes back to reading. When John arrives, he admits that he didn't get the shopping done and asks for cash. Sherlock directs him to his wallet, and John complains that he hasn't done anything all morning. When John asks about the Jaria Diamond case, Sherlock says that he wasn't interested and sent them a message. John notices that Sherlock is using his laptop to read an e-mail, and yanks it away.
John contemplates their bills and thinks he needs to get a job. He asks Sherlock for a loan, but Sherlock says he needs to go to the bank and walks out. John goes with him, and Sherlock meets with Sebastian, an old friend from university. Sebastian explains that there was a break-in at their former chairman's office, and the criminal crossed off the eyes on the portrait and painted graffiti on the wall nearby. The CCTV reveals someone managed to get in and out in less than 60 seconds, and the main door didn't open. Sebastian offers Sherlock a five-figure payment to find the hole in their security. Sherlock turns down the money and takes the case, but John collects the check anyway.
Sherlock goes over the crime scene and then steps out on the balcony, and studies the drop over 20 stories down. He then goes over the office, ducking and weaving to test lines of sight to the office. He finds one office and takes out the name tag of Edward Van Coon. As the leave, Sherlock tells John that the message was intended for someone, and there were only a few people who could see it. The message was left for someone who worked at midnight. Next, Sherlock goes to Van Coon's apartment and discovers that the owner has just moved in. He convinces a new resident to let them in and then uses her balcony to crawl over to Van Coon's apartment. Sherlock finds champagne bottles in the refrigerator and a variety of books, and a dead man on the bed, an apparent gunshot suicide.
As the police arrive, John wonders if it was a suicide like it appears. Sherlock reveals that Van Coon was being threatened, and removes a black paper flower from the corpse's mouth. DI Dimmock arrives and insists that it's a suicide. Sherlock disagrees and notes that Van Coon is left-handed, but a right-handed person shot him. Van Coon had been threatened, and was waiting for the killer. He fired a shot at the killer, and the bullet went out the window. Sherlock assures Dimmock that when they check the bullet in Van Coon's brain, it will be from a different gun.
Sherlock and John go to see Sebastian at a dinner meeting, and Sherlock insists on explaining how Van Coon was killed. Sebastian finally agrees to talk to them privately in the restroom, and insists that Van Coon had enemies. However, he admits that they don't usually commit murder. Sebastian's chairman calls to inform him that the police are calling Van Coon's death a suicide, but Sherlock insists they've got it wrong. Sebastian tells Sherlock not to get sidetracked.
That night, a journalist, Brian Lukis, is running through the streets and finally gets to his apartment. When he gets to his flat and locks the doors behind him, he stares at something in shock.
Andy is informed that Soo Lin has resigned her job. He goes to see her but gets no answer, so shoves a message in her door slot.
John applies for temp work at a surgery, and one of the doctors, Sarah, warns that he might be over-qualified. However, she agrees to hire him on.
Sherlock is contemplating the graffiti marks when John arrives. He shows John a case about a journalist who died in his flat, locked from the inside. Sherlock is sure it's the same killer. They go to the flat and Sherlock realizes that the windows are open, and that the victims didn't anticipate their killer would climb up. The killer entered the bank the same way, and Sherlock realizes they need to discover what connects the two men. A black paper flower is lying on the floor. He finds a book belonging to the West Kensington Library, checked out the same day the victim died. Going to the library, they find similar ciphers on the shelf where the book was located.
Sherlock figures that the cipher is an ancient codes and goes to talk to an expert: graffiti artist, Raz. Raz identifies the paint but doesn't recognize the symbols. He agrees to help, but the police arrive. Sherlock and Raz run off, leaving John holding the bag.
John returns home later and explains that he was arrested and given community service. Sherlock is busy trying to decipher the symbols and sends John to the police station to find Lukis' personal effects. Meanwhile, he goes to talk to Van Coon's personal assistant Amanda and retrace his steps. The PA notes that there is a gap in Van Coon's schedule on the day he died, and Sherlock observes she has an expensive bottle of hand lotion.
Dimmock gives John the journalist's diary, and notes that Sherlock is an arrogant sod.
Sherlock goes over Van Coon's receipts and confirms that on the day of his murder, he took a taxi into town, but took the underground back. He concludes that Van Coon delivered a heavy package, and finds a lunch receipt. Sherlock retraces Van Coon's steps and finds John there. John has used Lukis' diary and points out that he wrote down the exact address of a Chinese shop, the Lucky Cat Emporium. The two men go inside and John notices the same cipher on some of the labels. They go, and Sherlock explains that the ciphers are numbers in Hang Zhou, an ancient Chinese dialect. They're unaware that a woman is taking photos of them.
Jon and Sherlock stop at a restaurant, and Sherlock concludes that Van Coon and Lukis both smuggled something back from China, and dropped it off at the emporium. He figures that someone stole something, and the killer didn't know which one of them was the thief. As they talk, Sherlock notices a wet phonebook at Soo Lin's doorstep, despite the fact it hasn't rained for several days. He goes to investigate and finds an open window in the back. Sherlock climbs up, almost knocks over a vase, and sees wet spots on the carpet below. He realizes that someone else broke in and knocked over the vase, and then follows the relatively small footprints. When Sherlock finds a childhood picture of Soo Lin and a young boy, he realizes that the other person who entered the apartment is the acrobat... and he's still in the apartment. The killer chokes Sherlock unconscious while John paces outside, complaining and asking to be let in.
Sherlock finally opens the door and explains that they need to find Soo Lin. He notices the note that Andy left, with the museum address on it, and they go to see him. Andy explains that Soo Lin disappeared three days ago, and on her final afternoon she demonstrated a tea ceremony. They go to her locker, and Sherlock spots a statue with the numbers on it. As they leave, Raz arrives and takes them to an underground station with the same symbols in the same paint. They start searching nearby for more evidence and Sherlock locates the spray paint can in an empty yard. John follows the tracks and finds more ciphers on a brick wall.
Back at 221B Baker Street, Sherlock tries to break the code and notes that the numbers come in pairs. He wonders why they were painted near the tracks, and realizes that the painter is communicating with his people in the underworld. However, they need Soo Lin to break the code. They go back to the museum and tell Andy that Soo Lin's in danger, and ask about the tea pots. Sherlock realizes that an additional pot has been polished since they were there the previous day.
That night, Soo Lin breaks into the museum and performs the tea ceremony. Sherlock and John are waiting for her. She explains that she had to finish her work, and the killer will eventually find her. Soo Lin met the killer, Zhi Zhu, in China, and recognizes his signature. She shows them a foot tattoo she has of a Black Lotus Tong mark, and explains that their smugglers all bear that mark. She smuggled drugs for the Tong when her parents died and she had no other way to survive. Eventually she gave up the life and made her way to England, but Zhi Zhu came looking for her after five years. He asked her for help finding the stolen object, and appealed to her as his sister. Zhi Zhu works for Shan, the Black Lotus general, but Soo Lin refused to help her. When she came to work, she found the cipher and realized her life was in danger.
Sherlock shows Soo Lin the numbers, and she starts to explain that the numbers are based on a book. The lights go out and they realize that Zhi Zhu is there. Sherlock runs into the museum and the killer opens fire on him. John has Soo Lin lock herself in and then runs to help his friend. Zhi Zhu doubles back and confronts Soo Lin, who reaches out to him. A few seconds later, John hears the shot and runs back, and finds her dead, a Black Lotus paper flower in her hand.
Dimmock brings Sherlock and John in, and John demands to know when he's going to take the murders seriously. The inspector insists that they have no proof that the Tong is involved. Sherlock goes to the morgue and talks to Molly, and asks to see the corpses of Van Coon and Lukis. When Molly balks, he awkwardly compliments her on her hair and she agrees. Sherlock takes Dimmock there and shows him that they both have the Black Lotus tattoo on their feet. Dimmock is sufficiently convinced to let Sherlock access all of the books in both men's apartments, and gives them back the photo of the ciphers that they showed to Soo Lin. They check all the books that both men owned, trying to find a match. Once they have narrowed down the books, they try to figure out what word each code-pair indicates.
The next morning, John goes to work but the patients soon back up. Sarah goes to see John and discovers that he's dosed off due to lack of sleep. When he finally wakes up, Sarah explains that she took care of all his patients. He apologies and she wonders if he was on a date, and John says that he doesn't have a girlfriend.
At the flat, Sherlock tries to figure out which book that everyone would own. He checks dictionaries and the Bible without success, and then says that he and John should go out for air. John says that he has a date and Sherlock suggests that he go to the circus with two tickets . That night, John ends up taking Sarah to the circus, which is Chinese. He discovers that there are three tickets in Sherlock's name, and Sherlock arrives to say that he booked a third ticket. John is angry that Sherlock wouldn't let him have one night alone. However, Sherlock figures that Zhi Zhu is a member of the circus, and he needs John's help. However, John isn't interested in cutting his date short.
The performance begins with an escape act where a Chinese matron introduces a masked performer, who needs to escape his bonds before a crossbow fires. He escapes just in time, and John notices that Sherlock has slipped away. Next, an acrobat descends from the ceiling on silk robes and soars around the ring. Sherlock spots the man and figures he's the killer. The matron comes backstage and leaves with a cell phone, and Sherlock finds a spray paint can with paint that matches the ciphers. A swordsman attacks Holmes, and John notices the commotion. The attacker knocks Sherlock out through the curtains and John tackles the killer. The acrobat removes his mask and runs off, while Sarah grabs a stick and beats Sherlock's attacker unconscious. Sherlock confirms the man has a Tong tattoo on his foot.
Sherlock, John, and Sarah report to Dimmock, who tells them that by the time his men got there, the hall was deserted. Dimmock wants to know what he can show his superiors, but Sherlock admits that they don't know what the Tong is looking for.
Back at 221B, Sherlock realizes that they need to find the Tong's hideout, and the answer is in the ciphers. Sarah starts to go, but John insists that she stay. He tries to find some food in the flat, while Sarah annoys Sherlock with her questions. Mrs. Hudson brings up some food, while Sarah notes that two words have already been translated, from the photo that Dimmock brought back from the museum. Sherlock realizes that Soo Lin started to decipher the cipher, which said "Nine Mill," and that she had the book they need.
Sherlock tries to hail a taxi without success, and notices tourists using an A-Z London guide. He realizes that's the book the Tong is using.
Sarah suggests that she's had a busy night, and John says they should get takeaway. They make the call, and there's a knock on the door a few minutes later. The deliveryman demands to know where the treasure is. When John admits he has no idea what he's talking about, the deliveryman knocks him out.
Sherlock breaks the cipher, which says, "Nine mill for jade pin, Dragon den black, tramway." He goes back to the flat and discovers that Jon and Sarah are gone, and the room is covered in cipher graffiti.
John wakes up and finds Sarah tied up next to him. The matron from the circus is there, and believes that John is Sherlock. She checks his wallet and finds Holmes' debit card, Sebastian's check, and Sherlock's tickets. The woman introduces herself as Shan, the Black Lotus General, and prepares to shoot him. However, the gun has blanks, and Shan explains that they weren't really trying to kill him.
Sherlock grabs a map and finds the black tramway.
Shan loads her gun with real bullets and explains that they wanted Sherlock to find the Empress' hairpin. When John says he has no idea where it is, Shan puts Sarah in front of the crossbow from the circus, and explains that one of her people stole the hairpin. When John insists he doesn't know where it is, Shan cuts open the sandbag, which starts to fall toward the crossbow trigger. However, Sherlock calls out from the shadows, and then knocks out the Tong member she sends to find him. He warns that the bullet could easily ricochet, and then leaps out, knocks over a flaming barrel, and tries to free Sarah. Zhi Zhu chokes him, while the sandbag descends toward the trigger. John manages to get up and knock over the crossbow, and the bolt hits Zhi Zhu. Shan runs off and Sherlock frees her, while John assures her the next day will be different.
Later, Dimmock arrives and Sherlock suggests that he take credit. Dimmock is grateful, and admits he'll follow where Sherlock points him.
The next day, Sherlock and John go to the bank, and Sherlock explains that Van Coon was the thief, and that the man had a jar of hand soap at his apartment. Amanda uses the same brand of hand lotion at the office. Sherlock confronts Amanda and notes that she was having an affair with Van Coon. She ended it because she thought Van Coon took her for granted. Sherlock asks for her hairpin, and confirms that it's the Empress' hairpin, and Van Coon gave it to her as a gift and an apology without realizing its value. Amanda panics when she learns that it's worth nine million pounds.
Later, John and Sherlock have supper, and John realizes that Sherlock is frustrated that Shan escaped. He explains that the smugglers will pick up another book. John looks outside and sees a Chinese woman writing more ciphers on the neighboring walls.
Shan contacts her benefactor, "M", by instant messaging and thanks him for arranging her passage from London. She warns that his safety is compromised, but "M" assures her that they can't trace him through her. Shan insists that she won't reveal his identity... as a sniper kills her.