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TV commentator Stephen Boyd doesn't believe the official verdict of suicide in the death of a famed London psychiatrist. Boyd tries to get to the truth by studying a list of the shrink's patients. While interviewing three of these worthies (Jack Hawkins, Diane Cilento and Richard Attenborough), Boyd discover that each has a deep dark secret that the psychiatrist was privy to. The best-kept secret concerns the schizophrenia of the dead man's teenaged daughter (Pamela Franklin)--a fact that provides the key to mystery. The Third Secret originally featured Patricia Neal as one of the suspects, but her scenes were cut from the final release print.