English adventurer Harvey Landon agrees to help his old Oxford friend, Don Vargas, who is plotting the overthrow of a South American government. However, the Revolutionaries are without aircraft, and to obtain their supply, Landon volunteers to take a letter out of the country to a mysterious Mr. Delgado in New York. Complications begin as Landon is stalked on board the New York-bound oil-tanker by two agents intent on killing him and stealing the letter. How the adventurer thwarts his enemies makes a powerful and gripping tale.