Sherlock Holmes and Doctor Watson are summoned to Theobald Grange, the Warwickshire home of Lady Heminworth. Being of a nervous and superstitious disposition, her Ladyship lives in fear. Her husband and elder son were recently murdered, apparently by the ghost of a court jester who was executed on the site centuries before. The apparition has warned that she, too, is to die. Holmes rejects a supernatural explanation, although his adversary seems unaffected by gunfire and is able to take flight and disappear. The Great Detective brings his powers to bear, but still the killings continue...

Sherlock Holmes and Doctor Watson take on the case of Mr Avery Quill, whose daughter has mysteriously disappeared. The investigation has hardly begun when her body is discovered, and Mr Quill is himself murdered in the same manner. The killer leaves visiting cards, emblazoned with the head of the Gorgon, at the scenes of his crimes, and Holmes connects these to a strange club known as 'The One Hundred per Cent Society'. He learns little there, except that the members are all outcasts from noble families. The following morning a visitor arrives at Baker Street, to tell Holmes and Watson that the Society has a sinister purpose and that its leader has identified them as an obstacle to his plans which must be removed. Thus begins a battle with a new and merciless adversary, whose madness and thirst for revenge threaten to bring about the deaths of an apparently unconnected group of people. The Great Detective pits his skills against an enemy whose very appearance is uncertain.

The body lying on its back before us was unquestionably that of the man who had attempted to kill Holmes and I with explosives, at Glenbury Gardens. He wore the same dark clothing, although his hat was gone, and the hare lip was a livid scar on his pallid face. Strangely, it crossed my mind that what could be seen of his expression in death was softer than that he had worn in life.

A visit from a young woman with a guilty conscience causes Sherlock Holmes and Doctor Watson to investigate the arrest of her ex-fiancé. Imprisoned while awaiting trial for the murder of the lover of his wife-to-be, he seems certain to receive the ultimate punishment. When it is realised that the sister of the murdered man has also recently died in suspicious circumstances, Holmes turns his attention to another possibility. While in pursuit of information, their informant is killed nearby. A subsequent interview with a prisoner in Pentonville prison confirms the name of a likely suspect, a criminal known as ‘The Phantom Killer’ because of his ability to change his appearance and evade capture. Following the trail to Devon, both Holmes and Watson are surprised to find that their suspect lives there under another name, as a prominent and respectable citizen. The local police quickly make it clear that Holmes’ presence is resented, and dismiss his purpose as ridiculous. Nevertheless, after an initial encounter with the suspect, during which he appears friendly, charming and an unlikely adversary, several ‘accidents’ almost claim the lives of the duo. When they discover that their quarry has suddenly departed for London they follow in haste, to be eventually confronted by his murderous wrath as he boasts of killing ‘for pleasure and for profit’.

Holmes and Watson journey to the Lake District at the bidding of Squire Foley, who claims to be tormented by a man he believes he has killed in a duel. After an unsatisfactory conclusion they return to London, where they investigate the abduction of a woman in most peculiar circumstances. Next, Inspector Lestrade brings them news of queer goings-on in the National Gallery, before a perplexed priest requests their help with the curious behaviour of his colleagues. Early on, Watson realises that a common thread runs through these events, and Holmes sets out to bring to justice 'probably the most evil woman I ever hope to encounter'.

Inspector Lestrade brings Sherlock Holmes and Doctor Watson an account of six murders. The only fact common to them all is that the victims had all served terms of imprisonment for serious crimes. It appears that someone believes that they should have suffered execution, and has set out to take the law into his own hands. A note is found, signed 'The Justice Master', warning that he will not be deterred from his purpose, nor does he spare anyone who gets in his way. The inspector is angry with his superiors because he feels that their efforts to apprehend this man have been insufficient, hence his consultation with The Great Detective. The murders continue and Holmes, with the occasional interference of an Old Bailey solicitor, Mr David Grantly-Knight, resolves to bring the killer to justice. The Justice Master proves to be as elusive as he is merciless, and narrowly escapes a trap set by Holmes and Lestrade. Then comes an encounter near Plantain Castle.

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.