British secret agent Alfred Pettibone carrying a vital secret document is murdered on his way to deliver it in the USA. The British government turns to Holmes for help. He deduces that Pettibone converted the document to microfilm. Avoiding an assassination attempt, he hurries to Washington with Watson to retrieve it before it falls into the hands of an "international spy ring". Holmes is certain that the spies do not yet possess the document, as people who were in contact with Pettibone on his journey have been harassed. Pettibone's body was then delivered to Holmes by the spies as means to throw Holmes off the track.