One day, Sherlock receives an email from a university classmate who works at a bank and, accompanied by a financially strapped John, heads to the bank. The previous midnight, someone had broken into an office preserved in memory of the bank's former chairman and, within one minute, painted a symbol in yellow paint (part of the symbol was drawn directly across the eyes of the chairman's portrait on the wall, hence 'Blind Banker'). Sherlock deduces that the symbol was a message intended for a specific bank employee. They go to the employee's home, only to find him dead inside his locked high-rise apartment.
While Sherlock continues his investigation, John takes a temporary job at a clinic. The case develops further: Soo Lin Yao, who works at a museum, abandons her job and disappears. A news reporter is also found dead in his own locked home. Following the clues, Sherlock discovers more mysterious symbols painted in yellow. It turns out that both victims were involved in smuggling antiquities for an organization, and the symbols are a code based on a cipher called the "Suzhou Numerals" used for the smuggling operations.
Noticing anomalies at Soo Lin Yao's home, Sherlock climbs through a window to investigate and is attacked from behind. Sherlock and John go to the museum, where they find Suzhou Numerals on a statue. With the help of a graffiti pigment expert, they locate an entire wall covered with the code and eventually find Soo Lin Yao hiding in the museum. Soo Lin explains the smuggling activities of the Black Lotus gang and reveals that the Suzhou Numerals correspond to specific words on a particular page of a book. A gunshot rings out; they rush to investigate, but upon their return, Soo Lin is dead.
John invites Sarah from the clinic to the circus, but Sherlock tags along uninvited. The two cause a commotion at the circus, which is operated by the Black Lotus gang. By the time the police arrive, the gang members have fled. Sherlock deciphers the final message on the street outside, which reads: "Find the jade hairpin worth £9 million." Meanwhile, inside, Sarah and John are taken hostage by the gang. Sherlock arrives in the nick of time. During the ensuing chaos, the gang's assassin is accidentally killed. It is revealed that the bank employee had stolen the hairpin during a smuggling operation and given it to his lover. The fleeing leader of the Black Lotus gang communicates with Moriarty via computer, only to be shot dead.