A convicted thief in Dartmoor prison hides the location of the stolen Bank of England printing plates inside three music boxes. When the innocent purchasers of the boxes start to be murdered, Holmes and Watson investigate.

Basil Rathboneas Sherlock Holmes
Nigel Bruceas Dr. John H. Watson
Patricia Morisonas Mrs. Hilda Courtney
Frederick Worlockas Colonel Cavanaugh
Harry Cordingas Hamid
Carl Harbordas Inspector Hopkins
Holmes Herbertas Ebenezer Crabtree
Edmund Breonas Julian 'Stinky' Emery
Mary Gordonas Mrs. Hudson