Sherlock and Dr Watson receive a visit from Mary Morsten, who offers up a particularly cryptic puzzle for them to solve. Her father went missing six years ago, and since then, she has received a pearl for every year he has not reappeared. Now, the treasure's sender has requested a meeting, and Mary would like Sherlock and Watson to accompany her. Finding the mystery benefactor is only the start of this adventure that puts Sherlock and Watson hot on the trail of cold-blooded killers and thieves—and a lost Indian fortune.