Case Closed by Mark Posner is a comprehensive study of the assassination of President John F. Kennedy and an examination of the numerous conspiracy theories that have been propagated since that time on November 22, 1963. There have been numerous theories of conspiracy and the hypothesis has been forwarded that Lee Harvey Oswald was not the sole assassin. Some place him in the employ of the CIA, the KGB, the Cubans, or the Mafia. Posner debunks all of these theories and examines the numerous government documents that have become available and concludes that Lee Harvey Oswald acted alone in the assassination of President Kennedy.

Posner provides an in-depth examination of Oswald's life and activities leading up to the assassination. He looks at his unhappy childhood and his mental instability. Oswald was a self-taught and self-proclaimed communist even though he was never a card-carrying member of the Community Party. After being discharged from the Marines, he followed his own well-formulated plan to defect to the Soviet Union, where he lived for three years. The Soviets classified him as mentally unstable and assigned him to menial labor in Minsk. In Minsk, he met and married his wife Marina Prusakova and had their first daughter, June. Disillusioned with the Soviet Union, he and his family returned to the United States.

Oswald was mentally unstable and lived in somewhat of a fantasy world. He created false identities and continually lied to people, from boarding house landladies to government officials. He always tried to make himself appear important. He was physically and mentally abusive to his wife, who refused to move back in with him the night before the assassination. He had a history of being fired from menial jobs, even though he tried to provide for his wife and family.

Posner also examines the activities of Jack Ruby, who killed Oswald on Sunday, November 25, as he was being transferred to the sheriff's jail. Posner looks at the inconsistencies in the testimony and the factors that contributed to the multitude of conspiracy theories. His original conclusion that Lee Harvey Oswald acted alone has not changed in spite of the theories of Mark Lane, Jim Garrison, and others. He looks at many of the questions and theories and debunks them based on fact and science.