Contains spoilers for this episode and earlier episodes.
Sherlock's former university acquaintance Sebastian Wilkes asks him to examine a break-in at a London bank. Someone has left strange symbols on a portrait without being caught by the cameras. Sherlock identifies the intended reader, trader Edward Van Coon, and finds him dead in his locked flat. When journalist Brian Lukis dies in similar circumstances, the same markings turn up at a library he visited.
The victims' trips to China connect the deaths to an antiquities smuggling ring called the Black Lotus. At a museum, pottery expert Soo Lin Yao explains that she once belonged to the organisation and recognises its warnings. Her brother kills her before she can finish translating them. Sherlock knows the numbers refer to a book, but searching the dead men's collections does not reveal which one.
John's attempt to take his new colleague Sarah on a date becomes another part of the investigation when Sherlock joins them at a Chinese circus. Gang members later abduct the couple, mistaking John for Sherlock and demanding their missing treasure. Sherlock finds the key in a London A-Z guide and reaches them in time. The object is a valuable jade hairpin that Van Coon gave his secretary without knowing its worth. General Shan escapes, but her unseen contact ends their conversation with a gunshot.