Sometimes (like in its Wikipedia article) called a "documentary," the film features reenactments of a Texas murder in which the wrong man, Randall Dale Adams, was convicted and sentenced to death. These reenactments are based on the false testimony of various witnesses. The only version never reenacted is the one the film proves to be true--namely, that the murder, of a police officer, was done by David Harris, a juvenile with whom Adams had drank beer at a drive-in movie earlier in the evening of the shooting.
How did the investigation go awry? Pretty clearly the problem was that a police officer had been shot, and the law enforcement establishment--the police and the county attorney of Harris County, Texas--wanted an execution. Harris, however, was a juvenile. Adams was prosecuted because he was an adult and could be executed. Were it not for this strange and wonderful movie, he probably would have been. Directed by Errol Morris, the film's total effect is augmented considerably by the Philip Glass score. The Thin Blue Line is a significant contribution to the cinematic arts which has not, however, diminished the enthusiasm for capital punishment among the part of the population that just might vote for Rick Perry for president.