I was actually a little surprised when renowned documentary maker Errol Morris won his first Oscar this year for The Fog of War. Surprised in part because it had taken so long, but also because, for my money at least, The Fog of War was a lesser Morris film. Then again, the win made sense in the eyes of the Academy and its restrictive selection process - Morris himself had been deserving in the past, and the subject was modern American history and a man who was important in its shaping.

It must be irritating for any film-maker to have people look back at their early work and cite it as their best, suggesting by association that it's all been downhill from there, but when you've made a documentary as compelling, as generically adventurous, as beautifully constructed and as socially important as The Thin Blue Line relatively early in your career (it was Morris's third film), there is a level of inevitability to this. It's not that Errol Morris has made weak films since - far from it - it's just that The Thin Blue Line is genuinely great cinema, a seminal example of the modern documentary work at its most gripping and persuasive.

The film is an investigation of the case of Randall Adams, who was convicted in 1976 for the murder of Dallas patrolman Robert Wood, a crime he always protested his innocence of. Chief witness for the prosecution was young David Harris, who was with Adams on the night of the killing, having given him a lift after his car had run out of fuel. According to Adams, the two shared a few beers, some marijuana and a drive-in movie, then parted company after he had felt unable to respond positively to Harris's request for a bed for the night. Harris, however, claimed that he was riding in Adams' car when they were pulled over by Wood and his partner for a minor traffic violation, and that he watched on as Adams pulled a gun and killed the officer.

The project came together largely by chance. Morris's original intention was to make a film about Dallas psychiatrist James Grigson, who had earned the nickname 'Dr. Death' for the number of cases in which his testimony has been instrumental in sending the defendant to the electric chair. In the course of his research, Morris interviewed Adams and became interested in his case, but it was only when he spoke to Harris that he began to believe that the wrong man had been convicted of the crime. For Morris this was not the open and shut case the police were claiming it to be - indeed, their complete co-operation with the filmmaker was in no doubt partly due to their certainty that the only film that could really be made about the incident was one detailing Adams' obvious guilt.

The film unfolds like a detective story, and though it employs many of the familiar techniques of the documentary format, it applies them in ways that were strikingly different back in 1988 and, thanks to their under use since, still largely so today. More surprisingly, some of the standard iconography of the genre is completely thrown out - there is no voice-over, no textual scene-setting and no captions to identify the interviewees. This can initially prove a little disconcerting for those coming to the story cold, as they are required to pick up information as they go, often from almost off-hand comments. The film opens, for example, on an unnamed figure relating a story whose significance will take a good five minutes to become clear - we eventually identify him as Randall Adams because of something said not by him, but by David Harris. Other characters in the tale are similarly identified though the words of others or pertinent newspaper headlines.

In this sense the film plays more like a drama than a documentary, where a character's name and role in the narrative become evident through conversation and incidental detail (sharp-eyed viewers will spot Randall's name on his prison overalls). But this sets up an important rule regarding the relationship between the film-maker and audience that is essential to full appreciation of the film's complexity and subtleties - you have to pay attention, and I mean really pay attention. This extends to all aspects of the film, including an area that elsewhere is largely used for purely illustrative or even decorative purposes, that of dramatic reconstructions. Here they too initially seem to function as visually interesting accompaniments to the talking heads - stylishly lit and shot, they largely avoid faces and expressions and concentrate on often small and seemingly insignificant detail. But right from the start these sequences are part of Morris's storytelling technique, used to re-enforce or cast doubt on specific testimonies and plant unspoken ideas in our heads - many of them are repeated throughout the film, but with subtle changes that prompt the audience, almost subconsciously, to question what has been said. Thus the tail light of a Chevrolet Vega becomes a tail light of a Mercury Comet to both illustrate and cast doubt on their mix-up (very nicely emphasised by a Freudian slip from one of the investigating officers in interview); a highly stylised shot of a cast-aside milkshake at first seems to represent the spilled blood of the murdered officer, but provides later total recall on where his partner was when the shooting took place; shots of a wall clock are used to underline the shaky memory of one witness regarding the timeline of events; cigarettes stubbed out into an already busy ashtray suggest the length of Adams' initial interrogation; wide shots of cars passing the two cars are used to cast severe doubt on the claims of two witnesses that they got a good look at the driver. And these are just a few examples.

As the facts emerge and the witnesses pile up, the flimsy nature of the case against Adams becomes increasingly worrying. Local prejudice, the blocking of key evidence and unreliable witnesses all play their part, but Morris here proves the best defence lawyer Adams could have had, convincingly countering almost every fact presented for the prosecution, sometimes with the aid of the police themselves. At the centre is Adams, who tells his story calmly and persuasively, but is still clearly unable to believe the hand that fate has dealt him. His description of the tests carried out by the aforementioned James Grigson casts serious doubt on the good doctor's very credibility both as an expert witness and a noted psychiatrist, and the case itself reflects poorly on the small town Texas justice system, which comes across as determined to make the crime fit the man rather than impartially investigating the facts of the case.

Morris takes an initially even handed view of events, subtly and steadily undermining the police view with almost invisible skill, only really showing his hand in an obvious way through the inclusion of anecdotal stories from Judge Don Metcalfe and witness Emily Miller, which are gently mocked through the inclusion of B-movie footage and satirical music, music that elsewhere underscores the film like a mournful heartbeat, only rarely used for anything close to traditional dramatic effect. Created by the then not widely heard minimalist composer Philip Glass, it is such an essential component of the film's mood and structure that it is impossible to retrospectively picture a single scene without it.

Like all great detective stories, Morris saves his biggest twists for the final scenes. The timeline of events provide some of the surprises, but an unexpected reveal involving Harris's hands is beautifully timed, and the final tape recording, played over huge close-ups of the recorder and subtitled for clarity, packs a devastating punch - on one screening of the film I overheard one of the audience angrily whisper "Bastard!" to herself. Even when you know the full facts of the Adams case, this is still a mother of a note to go out on.

As a documentary work, The Thin Blue Line scores a bulls eye on all counts. A remarkable cinematic achievement, as a rethink of the documentary form it still has few peers, and for Morris the filmmaker it was the work that most effectively shaped the techniques he has continued to employ to this day. But the film's greatest achievement is its successful challenge of a wrongful conviction and powerful illustration of the failings of the justice system, ultimately proving instrumental in overturning Adams' conviction and setting him free. Other film-makers have followed in Morris' wake, and TV series such as the BBC's Rough Justice have prompted the re-examination of a number of suspect convictions, but back in 1988 Morris trod where few film-makers had gone before. We are left with a sense of gratitude for what his film achieved, admiration for his skills as an investigative documentarian, but a feeling of cold horror that, were it not for the determined efforts of the Morris and his team, an innocent man would have spent the rest of his life in jail for a crime that even some of those who convicted him must have suspected he did not commit.

Sound and Vision

One of the most important aspects of this release is the restoration (on video at least) of the film's original 1.85:1 aspect ratio. Previously only available on home video format in a cropped 4:3 print (which was also screened by Film Four - and Channel Four were one of the funding companies behind the film), in the case of The Thin Blue Line this was not just as aesthetic irritation, it genuinely affected the clarity of certain sequences. Morris uses headlines and extracts from newspaper stories as information clarifiers, but frames the story extracts in such a way that the words he wants you to read are shown in their entirety, while words on lines above and below are cut in half. This is a very effective method of steering the eye to a particular word or phrase without the use of traditional post-production highlighting, but in the 4:3 prints was completely nobbled, with the start and end of all lines, headlines included, cut off. Now the film can be seen as it was meant to be and benefits greatly from it.

The transfer is anamorphic and for the most part very good, given the restrictions of the original material. Some grain is evident throughout and colours and definition vary a little in the interviews, as does the contrast and black levels, but these sequences were conducted on location in probably less than ideal conditions, and when Morris has full control of the imagery, as in the reconstructions, the picture quality is strong.

The sound is Dolby 2.0 surround, and though all of the interviews are centrally located mono, Philip Glass's gorgeous score is more widely spread. Given the sometimes strong Southern accents, clarity is good, but it's nice to have a subtitle option if your ear is not tuned to that particular drawl.