The English bookseller David Tristram visits his friend Sherlock Holmes in Montpellier, southern France, where he is engaged in his chemical experiments away from the hustle and bustle of the city. Together, they pay a visit to an antiquarian bookshop in Nîmes. There, Holmes is offered nothing less than the diary of his grandmother. However, the owner of the manuscript, a lawyer, is found murdered in Paris. Holmes and Tristram then travel to the capital but find no trace of the diary in the deceased's estate. Holmes discovers that the records of his relative contain clues to a mysterious treasure that Marie Antoinette managed to hide before her imprisonment and execution by guillotine. But not only Sherlock Holmes is intensely interested in the old manuscript, which makes his search so dangerous.