When Mary Morstan consults Sherlock Holmes on a curious mystery, he is intrigued and anxious to help her. The young governess explains that since the disappearance of her father ten years earlier, she has received a valuable pearl each year from an unknown donor. The sender then asks to meet Mary and she realises that she needs an escort. Holmes and Watson accompany Mary to her encounter with the stranger, who tells them a tale of secrecy, hidden treasure and sudden death. But there is a further death and only the brilliant Sherlock Holmes has the intellect and skill to uncover the real villain.