During his farewell address in 1961, outgoing President Dwight D. Eisenhower warns about the build-up of the military-industrial complex. He is succeeded by John F. Kennedy as president, whose time in office is marked by the Bay of Pigs Invasion and the Cuban Missile Crisis until his assassination in Dealey Plaza, Dallas, Texas, on November 22, 1963. Former US marine and suspected Soviet defector Lee Harvey Oswald is arrested for the murder of police officer J. D. Tippit and arraigned with both murders but is killed by nightclub owner Jack Ruby. New Orleans District Attorney Jim Garrison and his team investigate potential New Orleans links to the JFK assassination, including private pilot David Ferrie, but their investigation is publicly rebuked by the federal government and Garrison closes the investigation.
The investigation is reopened in 1966 after Garrison reads the Warren Report and notices what he believes to be multiple inaccuracies. Garrison and his staff interrogate people involved with Oswald and Ferrie. One such witness is Willie O'Keefe, a male prostitute serving five years in prison for solicitation, who says that he witnessed Ferrie talking with a man called "Clay Bertrand" about assassinating Kennedy, and that he briefly met Oswald. Garrison and his team theorize Oswald was an agent of the CIA and was framed for the assassination.
In 1967, Garrison and his team talk to several witnesses, including Jean Hill, a teacher who says she witnessed a gunman shooting from the "grassy knoll", a small hill, that Secret Service threatened her into saying three shots came from the Texas School Book Depository from which Oswald was said to have shot Kennedy, and her testimony was altered by the Warren Commission. Garrison's staff also test fire an empty rifle from the Depository and conclude that Oswald was too poor a marksman to make the shots, and that there was more than one shooter. Garrison comes to believe that "Bertrand" is really New Orleans businessman Clay Shaw. Garrison interviews Shaw, who denies having ever met Ferrie, O'Keefe or Oswald.
Some key witnesses become scared and refuse to testify while others, such as Ruby and Ferrie, die in suspicious circumstances. Before his death, Ferrie tells Garrison that there was a conspiracy to kill Kennedy. Garrison meets a high-level figure in Washington D.C. who identifies himself as "X". He suggests a coup d'état at the highest levels of government, implicating members of the CIA, the Mafia, the military-industrial complex, Secret Service, FBI, and then-Vice President Lyndon Johnson as either co-conspirators or as having motives to cover up the truth of the assassination. X suggests that Kennedy was killed because he wanted to pull the United States out of the Vietnam War and dismantle the CIA. X encourages Garrison to keep digging and prosecute Shaw. Soon afterward, Garrison indicts Shaw with conspiring to murder Kennedy.
Garrison's marriage is strained when his wife Liz complains that he is spending more time on the case than with his own family. After a sinister phone call is made to their daughter, Liz accuses Garrison of being selfish and attacking Shaw only because of his homosexuality. Some of Garrison's staff begin to doubt his motives and disagree with his methods, and leave the investigation. One of them, Bill Broussard, is later revealed to have been an insider for the FBI for some time, and even plays a peripheral, undisclosed role in what seems to be an attempt to kidnap, murder or otherwise scare Garrison. In addition, Garrison is criticized in the media as wasting taxpayer money to investigate a conspiracy theory. Garrison suspects a connection with the assassination of Martin Luther King Jr. and the assassination of Robert F. Kennedy.
Shaw's trial takes place in 1969. Garrison presents the court with a dismissal of the single-bullet theory, proposing a scenario involving three assassins firing six shots and framing Oswald for the murders of Kennedy and Tippit, all for the purpose of installing Johnson as president so he could escalate the war in Vietnam and enrich the defense industry. However, the jury acquits Shaw after less than one hour of deliberation. While his prosecution has failed, Garrison wins his wife and children's respect for his determination, and so repairs his relationship with his family.
Kevin Costner as Jim Garrison
Kevin Bacon as Willie O'Keefe
Tommy Lee Jones as Clay Shaw / Clay Bertrand
Laurie Metcalf as Susie Cox
Gary Oldman as Lee Harvey Oswald
Michael Rooker as Bill Broussard
Jay O. Sanders as Lou Ivon
Sissy Spacek as Liz Garrison
Joe Pesci as David Ferrie
Beata Poźniak as Marina Oswald Porter
Jack Lemmon as Jack Martin
Walter Matthau as Senator Russell B. Long
Donald Sutherland as Mr. X
Ed Asner as Guy Banister
Brian Doyle-Murray as Jack Ruby
John Candy as Dean Andrews Jr.
Sally Kirkland as Rose Cheramie
Wayne Knight as Numa Bertel
Pruitt Taylor Vince as Lee Bowers
Tony Plana as Carlos Bringuier
Vincent D'Onofrio as Bill Newman
Dale Dye as General Y
Lolita Davidovich as Beverly Oliver
Ellen McElduff as Jean Hill
John Larroquette as Jerry Johnson
Willem Oltmans as George de Mohrenschildt
Tomas Milian as Leopoldo
Gary Grubbs as Al Oser
Ron Rifkin as Mr. Goldberg / Spiesel
Peter Maloney as Colonel Finck
John Finnegan as Judge Haggerty
Wayne Tippit as FBI Agent Frank
Jo Anderson as Julia Ann Mercer
Bob Gunton as News Anchor
Frank Whaley as Imposter Oswald
Jim Garrison as Earl Warren
Martin Sheen as Narrator
The film examines the investigation into the assassination of John F. Kennedy by New Orleans district attorney Jim Garrison, who came to believe there was a conspiracy to assassinate Kennedy and that Lee Harvey Oswald did not act alone.