Sherlock Holmes is hired by a lady who is being harassed by Milverton, a notorious blackmailer who has ruined many people and caused many grief. Though he is determined to stop him, Milverton is very crafty and Holmes is unable to find the evidence necessary to put him away. Finally, Holmes and Watson decide on a daring plan to retrieve their client's incriminating letters. While there, they witness a crime that forces Holmes and Watson to make a momentous choice.