Someone has broken into a big London bank and painted a strange Chinese cipher on the wall. Across town, the same cipher turns up at a library. Two men connected with each of these sites are later found dead: a Hong Kong trader and a freelance journalist. Given the height and security of the various buildings involved, only a killer who can scale sheer walls could have done it. And only the master of deduction can solve it: Sherlock.
Sherlock and Dr. Watson soon establish that both men often traveled to China and both met their end shortly after visiting the Lucky Cat Emporium in London. The meaning of the cipher is more baffling, and Sherlock contacts a friendly graffiti artist for help cracking this puzzle. Also helpful is a fugitive museum curator, who herself has received the ominous cipher. And eagerly sought is "the book that everybody owns," whatever that is, which holds the definitive solution to encrypted Chinese messages appearing throughout London.
Complicating the plot, Watson has found a part-time job at a medical clinic and has romantic designs on his boss, Sarah. Their first date — to the circus — turns into a circus of sorts when they confront a life-threatening peril. But not as perilous as the fate that awaits them after Watson is delivered the lethal cipher, under the killer's mistaken impression that he is Sherlock.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
Why would someone break into a London investment bank only to splash a strange Chinese cipher across the eyes of the former chairman's portrait — rendering him blind, so to speak? That's the question an executive at the bank pays Sherlock handsomely to answer.
The mystery deepens when the intended recipient of the message, a Hong Kong trader at the bank, is found murdered in his apartment. As with the graffiti incident, whoever did it is able to scale buildings and penetrate locked spaces like Spider-Man. Then, across town, a freelance journalist is shot dead, apparently by the same ghostly intruder. And in a place where the journalist can't help but see it: the same enigmatic Chinese cipher.
But what does the cipher mean? In a trip to London's Chinatown, Sherlock and Dr. Watson find evidence that it is part of an ancient Chinese numbering system. They further surmise that both the banker and journalist were delivering smuggled Chinese antiquities to an underworld dealer at the Lucky Cat Emporium in London, and that one of them must have stolen part of the loot.
Next, Sherlock is attacked by a stealthy assailant when he searches an apartment above the Lucky Cat, belonging to Soo Lin Yao, a missing curator at the National Antiquities Museum. Sherlock and Watson go to the museum where they find the cipher, spray-painted in the storage area. After finding more ciphers in another part of London, representing different numbers and presumably a more complicated message, they return to the museum and discover Soo Lin in hiding.
Soo Lin explains that in China she worked for a smuggling operation called the Black Lotus. Fleeing to London, she thought she had escaped their control, but she was contacted by her brother, a hit man known as the Spider, who asked her for help locating the stolen treasure. She refused and is now on the hit list herself. But before she can tell Sherlock and Watson the name of the book that holds the key to the cipher, the Spider appears. In the melee that follows, Soo Lin is shot dead.
The next evening, Watson arranges a date with Sarah, his boss at his new part-time job as a clinician. At Sherlock's insistence, the three go to the Yellow Dragon Circus, on Sherlock's hunch that the troupe's acrobat may be the Spider. Backstage, Sherlock finds spray paint of the type used for the ciphers. He is discovered and attacked by a sword-wielding performer. Wondering what she has gotten herself into, Sarah manages to save the day.
Back on Baker Street, Sherlock happens on the secret code book when he runs into tourists who are holding London A-Z, a street guide that everyone in the city owns. By turning to the page and line given by the Chinese cipher, he finds "Deadman" as the translation for the original message. The other message turns out to be a call to action about the stolen treasure.
Meanwhile, Watson and Sarah have been kidnapped by the circus troupe — members of the Black Lotus all — under the mistaken impression that Watson is Sherlock and knows where the stolen treasure is. Based on his decoding of the cipher, Sherlock finds the hideout and appears just in time to save Sarah from an arrow through the heart, which impales the Spider instead. The group's ringleader escapes, but she is later punished with death by her bosses.
And the stolen treasure? It is an antique jade hair pin, worth nine million pounds, being worn unwittingly by the dead banker's secretary and mistress.