For this is not the first mysterious item Mary Marston has received in the post. When a beautiful young woman is sent a letter inviting her to a sinister assignation, she immediately seeks the advice of the consulting detective Sherlock Holmes. Now it appears the sender of the pearls would like to meet her to right a wrong. Every year for the last six years an anonymous benefactor has sent her a large lustrous pearl. 2010. But when Sherlock Holmes and his faithful sidekick Watson, aiding Miss Marston, attend the assignation, they embark on a dark and mysterious adventure involving a one-legged ruffian, some hidden treasure, deadly poison darts and a thrilling race along the River Thames.