Holmes and Watson attempt to thwart the schemes of the infamous blackmailer Charles Augustus Milverton from blackmailing Lady Eva Blackwell. But things do not seem as easy as they do and Holmes must sacrifice his morality and his honourable career to stop Milverton from wrecking her marriage.

American heiress Henrietta Doran (known as Hatty) marries Lord St Simon, but she vanishes without trace on her wedding day, leaving London agog. Later, the police find her wedding dress in the Serpentine, a lake in a London park, with a note in the pocket of the dress. While investigating the case, Holmes suffers from nightmares, which he sees as a factor in the mystery.