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.

Dr Trevelyan calls on Holmes on behalf of his friend Blessington, after a Russian father and son vanish during a consultation. Blessington knows of no one who wishes him ill, but he is found hanged the next day. Holmes investigates, with advice from Watson.

An obscure family document containing an enigmatic ritual holds the key to a hidden treasure if Holmes can decipher the mystery.

Holmes is curious about a series of attacks on figures of the French Emperor Napoleon Bonaparte around London. One such incident leads to a violent death, and he investigates. Lestrade sees an Italian connection and proposes to focus Scotland Yard's attention on the Italian quarter of London, but Holmes is more interested in Chiswick.