The fourth series of the show commences with Jamie Theakston learning about the former German state executioner Johann Reichhart, who went from dispatching enemies of the Nazis to executing Nazi war criminals.

Jamie Theakston travels to Rome to meet church officials, expert archivists, scholars and authors to uncover some of the historical treasures stored in the Vatican Archives.

Jamie Theakston investigates a shadowy, secret department within the Bulgarian government suspected of carrying out assassinations in nine other countries during the Cold War.

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An examination into the murder of Benito Mussolini and possible motivations behind it other than the official story. Theories include his ultimate uselessness to Hitler and the supposed existence of incriminating letters from Churchill.