An unhappy wife learns that her husband, Colonel Barclay, cheated her earlier fiance, leaving him disabled and in exile. After a fierce argument between them, the Colonel's dead body is found next to his unconscious wife in a locked room with no key, and with curious animal tracks outside the window. Nancy Barclay remains in a coma.Holmes investigates and finds a story of betrayal and a mystery worthy of his powers.

A plumber, John Horner, is arrested just before Christmas and charged with stealing a famous jewel, the Blue Carbuncle, from Lady Morcar. He denies the theft - but he has a criminal record, and all the evidence is against him. After Holmes's doorman finds the Carbuncle in a Christmas goose, Holmes uses an old hat to track the goose to a poultry dealer, then traps the servant who stole the jewel and framed the plumber. The Blue Carbuncle has a dark history, and the stone features in the programme's opening credits with turbulent scenes from its past.

Violet Hunter consults Holmes about Edward Rucastle's offer of a job. He wants her to cut off her hair before coming to work for him as a governess. Holmes sees a sinister motive and investigates.