Over the last century there have been four successful presidential assassinations and a multitude of failed attempts. But do we know the real story behind these assassination plots? We'll look behind the headlines to the conspiracy theories that surround the most shocking murders in American History. What do John Wilkes Booth, Leon Czolgosz, Charles Guiteau, Lee Harvey Oswald and John Hinckley Jr. all have in common? According to experts, they were all mentally unstable. While this may explain their deranged obsession to kill the President, others believe this is evidence that all were patsies to a larger conspiracy. Former secret service agents reveal the secret methods used to safeguard the President of the United States. Bill O' Reilly reveals the elaborate conspiracy behind the murder of President Abraham Lincoln. And bestselling author Ronald Kessler examines whether an assassin could even infiltrate the Secret Service itself.