Frank Horrigan is a Secret Service Agent. He currently does mostly undercover work. He goes to check on a report about a person who is threatening the life of the President. When he goes to the man's apartment, he finds clippings and photos of various assassinations throughout history. When he does a background check, he discovers that the man's identity is false. So he goes back to the apartment to get him, but when he goes in there, he finds the apartment cleaned out except for a photo of him when he was in Dallas, November 1963, protecting Kennedy. Later he gets a phone call from the man and tells him that he plans to kill the President and is daring Frank to stop him. When Frank reports to his superiors, he finds that there are some people like the agent in charge of protecting the President and the Chief of Staff, who think that Frank is letting his failure to protect Kennedy cloud his judgement. But as Frank chases him down, he discovers that he is not dealing with a nut but with a well organized individual.