Holmes is commissioned by Sir James Damery who is acting on behalf of a mysterious client of great importance. Sir James is concerned about Miss Violet Merville who is besotted by the evil Austrian, Baron Gruner, who has already murdered one of his wives and abused and ill treated countless other women. His reputation as a Don Juan has preceded his arrival in London and Holmes is already familiar with his scandalous reputation.
Holmes and Watson have six weeks in which to break up Miss Merville's relationship with Baron Gruner before she takes the final fatal step and marries him. They find that they have an extremely dangerous opponent on their hands, one who will stop at nothing to add another female victim to his collection.