On the eve of World War II, England's King George VI and Queen Elizabeth tour the Canadian provinces and then, at the invitation of President Franklin D. Roosevelt, cross the border at Niagara Falls and enter the United States. Their unstated mission is to cement ties between Britain and America at a time when Hitler, the Nazi dictator, is determined to conquer Europe.
The Federal Bureau of Investigation receives intelligence that an assassination attempt will be made on the lives of the royal couple. Suspicion falls on two Irish brothers who have just arrived in New York City. Both have histories as rebels opposed to British rule in Ireland.
A third brother, an Irish-American who is deputy managing editor of an upstate New York newspaper, undertakes the task of discovering who is behind the plot. The story reaches its climax when the royal couple motors to Hyde Park, where they will be the weekend guests at the estate of President and Mrs. Roosevelt. The President has planned an all-American picnic at which the royal couple will be served hot dogs and beer.