A British secret service operative, carrying top-secret microfilm from England to Washington, disappears while traveling to his destination. Fearing for his safety just before his disappearance, he passes the microfilm, ingeniously hidden, to another passenger on the train without her knowing. The agent is reported missing and Sherlock Holmes is called in to investigate.

Filmed during World War II, Sherlock Holmes in Washington pits Holmes and Watson against Nazi enemy agents. The British government asks Holms and Watson to go to Washington to recover the missing documents before they fall into the wrong hands, which would be disastrous for England and her allies. Holmes is up against an international ring of spies in a race against time to piece together the clues and discover the whereabouts of the microfilm before it is to late.