Sherlock Holmes and Doctor Watson are visited by Mr Josiah Endicott, an injured enquiry agent who believes that three recent murders are connected.

He explains that he was attacked shortly after interviewing a friend of one of the victims and warned to discontinue his investigation. A remark of one of his assailants appeared to confirm that the murders were committed with a common motive and, as his client is clearly unable to proceed, Holmes agrees to assume the case.

The duo call upon Miss Daisy Scanlon, the young woman visited by Mr Endicott before he was injured, who was distressed but of some assistance. Returning to Baker Street, it is not long before a message from her arrives requesting a further meeting. Holmes realises at once that the appointment is a trap, but they set out to see what can be learned after ensuring that they are armed. They narrowly escape with their lives, immediately revisiting Miss Scanlon to discover that she has suffered a fatal accident in her home. Although it is not apparent, Holmes deduces that she has been murdered.

Consulting the extensive notes left by his client, Holmes then conducts a series of interviews. These lead him to believe that the murders are linked to The Experience Club, an establishment barred to all except gentlemen who have performed extraordinary feats. There he recognises agents of Imperial Germany, and eventually uncovers a plot to assassinate the Queen.

It transpires that German agents have abducted a Member of Parliament, in order to gain information, and also his daughter as an incentive to gain his co-operation. Holmes knows, despite discouragement from Scotland Yard and his brother Mycroft, that these incidents also must be addressed, to prevent a blow to our country that would devastate the Empire.