Basil Rathbone, the definitive Sherlock Holmes, stars in his last installment in the classic mystery series, with Nigel Bruce as his trusty sidekick Watson. When a prison inmate makes three cheap music boxes that are sold at auction, an old school friend of Dr. Watson's purchases one, later turning up murdered and robbed of his acquisition. The detectives soon recognize the boxes are not what they seem, and set to work tracking down the other two boxes before the killer does. They uncover evidence of a counterfeit ring, as well as competition in the form of the beautiful Mrs. Hilda Courtney (Patricia Morison), who seems to want to get her hands on the music boxes as well. The plot thickens as Holmes hones in on the truth in this classic based on the novels by Arthur Conan Doyle.