Executioner, The (1970, U.K.)

When a covert operation in Vienna goes awry, British Intelligence operative John Shay (George Peppard) suspects his colleague, Adam Booth (Keith Michell), may be a double agent. Despite help from his girlfriend, Polly (Judy Geeson), a clerk at MI6, Shay fails to convince his superiors of his theory. Undeterred, Shay learns from scientist Philip Crawford (George Baker) that Booth has been trying to steal top-secret documents. Appointing himself executioner, Shay kills Booth and then assumes his identity to obtain conclusive evidence that Booth was a traitor, only to discover the dead man’s wife (Joan Collins) is now Crawford’s mistress, and Soviet Intelligence has set him up.