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.

Judy Wilson (Gale Storm), feeling neglected because both of her parents are working in defense plants, meets and falls in love with Danny Chester (Jackie Cooper), who enlists in the Navy and is sent to San Diego for training. She accepts an invitation to go on a ride to San Diego with her friends Herb (Neyle Morrow), Opal (Evelyn Eaton)and Jerry (Jimmy Zahner) but doesn't know the car has been stolen.