Documentary concerning the murder of a Texas police officer who had stopped a car for a routine traffic citation. The police are presented with two suspects, one a local underaged boy with a criminal record (David Harris, a boy who returned to his hometown boasting that he had murdered a policeman) and the other a 38-year-old taciturn drifter with no criminal record whatsoever (Randall Dale Adams). The documentary presents testimony suggesting that the police altered, fabricated, and suppressed evidence to convict the man they wanted to be guilty, in spite of evidence to the contrary. They succeded, but with the help of Errol Morris, the film's director (who had, incidentally, spent some years before the filming as a private investigator), the man they convicted was finally granted a retrial and released, after 11 years in prison.
The title comes from the prosecutor's comment to the trial judge that the police are the "thin blue line" separating society from anarchy.