Whilst trying to retrieve an indiscreet letter which could damage her husband's position, from the blackmailer Fournaye,politician's wife Lady Hilda Trelawney Hope is the innocent witness to the man's murder. No lesser person than the Premier of Britain approaches Holmes to save Lady Hilda's reputation and her spouse's career.
Sir Henry is apparently attacked and killed by the hound but the victim is actually an escaped convict, Seldon, Mrs. Barrymore's brother, to whom she had given old clothes of Henry's and the dog, recognizing the scent from the boot stolen in London, had gone for him. Watson meets a disguised Holmes, living rough on the moors in order to survey the likely suspects. He believes he has seen the man who controls the hound and sets a trap for him with Sir Henry as its bait.
Mr. Melas comes to see Holmes. He was hired by a man called Latimer as a translater but the job became extremely sinister as he was hired to interpret to a fellow Greek, who was clearly being kept in Latimer's house against his will. A woman called Sophy, who turns out to be the prisoner's sister, is also involved.
A Foreign Office employee who had an important Naval treaty stolen whilst he was on duty has become sick with worry and retired from his job, staying with his fiancee and her brother. Watson is concerned for him and brings in Holmes, who is in time to thwart an attempted burglary from the man's sick room.
Wealthy Neil Gibson becomes Holmes' latest client. He is unhappily married and fond of Grace Dunbar, his children's governess. His wife was found shot in the head at Thor Bridge, and Grace, who is accused of the murder,went to the bridge to meet her at the time of the death.
Reginald Musgrave, who comes from a family with a long and noble history, asks for Holmes' assistance when his womanizing butler mysteriously disappears after Musgrave has caught him trying to locate a map of instructions, known as the Musgrave Ritual, which will allegedly unearth royal treasure, hidden in the Civil War.
Holmes helps Inspector Hopkins investigate the murder of 'Black' Peter Carey, a drunken, aggressive ex-sailor, harpooned to death in his log cabin in the woods. Peter seems to have known his killer as he had been drinking with them. A tobacco pouch with the initials P.C. is found but Carey did not smoke. A book with the initials J.H.N. leads Hopkins to arrest John Neligan, who was certainly in the cabin at some time but Holmes reckons he was too feeble a man to wield a harpoon. So who is P.C?
Mr. John Scott Eccles tells Holmes how he went to stay overnight at Wisteria Lodge as the guest of Spaniard senor Garcia but in the morning everyone had gone. Senor Garcia is dead and Holmes travels to Surrey to assist Inspector Baynes in the murder enquiry. Baynes believes that Garcia's servant is to blame but Holmes is more interested in a neighbour, Henderson, and his English governess, Miss Burnet, who are not what they first appear to be.
Holmes is engaged by John Mason, groom to Sir Robert Norberton, whose horse is to run in the Derby in a few days. Mason has noticed many things wrong at Shoscombe Old Place, such as Norberton's meeting a stranger in the family vault, the discovery of bones in a furnace, and the distant behaviour of Sir Robert's sister, who has refused to acknowledge her favourite horse and given away her dog. Holmes reveals a plot which, rather than criminal,is a desperate bid to buy time.
Miss Violet Smith is hired as a governess to the kindly widower Mr. Carruthers at his country house. She is unsettled by the presence of a rude house guest and even more so by a solitary cyclist who appears out of nowhere to follow her along lonely lanes, disappearing just as suddenly. She asks Holmes to investigate.