Holmes has been following the diamond thief, Count Negretto Sylvius, and is convinced that the man is going to try to kill him. The detective has therefore set up a clever visual trick at 221B Baker Street that should allow him to catch the count and not get killed. But will it be enough? And will Sherlock Holmes and Dr Watson be able to track down the precious stone? 'The Adventure of the Mazarin Stone' is part of "The Case-Book of Sherlock Holmes".