Based on the non-fiction JFK assassination book "Legacy of Secrecy," written by Lamar Waldron and Thom Hartmann, DiCaprio will play FBI informant Jack Van Laningham, who obtained the confession of Mafia godfather Carlos Marcello to having ordered JFK's assassination. As part of a dangerous and long-secret undercover operation, the FBI positioned Van Laningham to become the confidant of Marcello, who ruled organized crime in Louisiana and most of Texas for decades.