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 – and I know I'm flying in the face of popular opinion here – 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 had an important role 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 that he was innocent 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 when his own car ran 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 film in its present form came together largely by chance. Morris's original intention was to make a documentary about Dallas psychiatrist James Grigson, who had earned the nickname 'Dr. Death' for the number of times his testimony has been instrumental in sending defendants 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 at length 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 absolute certainty of Adams' 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 to the norm back in 1988 and still stand out from the crowd today. More surprisingly, some of the standard conventions of the genre are 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 the film unfolds, often from almost off-hand remarks. 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 not though his own words, but because of something said by David Harris in a seperate interview. Other characters in the story are similarly identified not by what they say but by 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 may spot Randall's name on his prison overalls). Such an approach means that little is laid out on a plate, and requires the viewer to pay close attention to every aspect of the film if they are to fully appreciate its subtleties and complexities. This includes a component that in other films is usually employed for illustrative or even decorative purposes, that of dramatic reconstructions. Intitially that's exactly how such sequences seem to be functioning here, as visual reconstructions of tales told by the interviewees, but right from the start the become part of Morris's storytelling technique, used to re-enforce or cast doubt on specific testimonies and plant unspoken suggestions in the the minds of the audience. Stylishly lit and shot, they largely avoid revealing faces and expressions and focus instead on often small and seemingly insignificant detail, and as many of them are repeated throughout the film they undergo a subtle metamorphosis that prompt us – almost subconsciously – to question the words they are seemingly designed to illustrate. Thus the tail light of a Chevrolet Vega becomes a tail light of a Mercury Comet to cast doubt on a police mix-up, which is nicely underscored by a Freudian slip by one of the investigating officers. A highly stylised shot of a cast-aside milkshake at first seems to represent the spilled blood of the murdered officer, but later prompts total recall regarding 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, while cigarettes stubbed out into an already busy ashtray potently suggest the length of Adams' initial interrogation. Elsewhere, wide shots of cars passing the scene of the shooting are used to cast severe doubt on the claims of two witnesses that they got a good look at the driver. And this is just a sample.

As the information and witnesses pile up, the flimsy nature of the case against Adams becomes increasingly troubling. 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 himself, who tells his story calmly and persuasively and 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 credibility both as an expert witness and a noted psychiatrist, and the case itself reflects poorly on the small town Texas justice system, whose enforcers appear to have been 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 case against Adams with almost invisible skill, only openly showing his hand in the inclusion of anecdotal stories from Judge Don Metcalfe and witness Emily Miller, both of whose testimonies are gently mocked through the use of B-movie footage and satirical music. Elsewhere the score, created by a then little known minimalist composer named Philip Glass, is a crucial component of the film's structure, underscoring the narrative like a mournful heartbeat, only rarely used for anything close to traditional dramatic effect.

Like all great detective stories, Morris saves his biggest twists for the final scenes. The timeline of events provides 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 an audience member 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. As a rethink of the documentary format 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 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 wlaked 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 that he did not commit.

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 – Channel 4 were one of the funding companies behind the film), in the case of The Thin Blue Line this was not just an aesthetic irritation, but genuinely affected the clarity of certain sequences. Morris uses headlines and extracts from newspaper stories as information clarifiers, but framed these in such a way that they sometimes run for almost the full width of the screen, steering the eye to particular words by cutting those not relevant in half at the frame edge. But in the 4:3 prints this was completely nobbled, as the front and tail ends of all headlines were effectively cropped and the key words were rendered as incomplete as those were were streered to ignore. 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.

There is only one real extra, a single episode from Errol Morris's 2000/2001 TV series First Person that is also available on three disk DVD set from MGM. "Mr. Personality" – First Person TV Episode (27:45) is the first episode of series 2 and consists of an interview with forensic psychiatrist Michael Stone, as his muses on the nature of evil and the workings of the human psyche. This is very much a precursor to The Fog of War in being centred around a single interview with occasional interjections from Morris himself, and through the use of archive film material to accompany Stone's words. It's also very much a Morris work in the way it is constructed to allow Stone to have his say and yet still make us wonder if he has not sold himself theories that he is uncertain of. Less enthralling, however, is the use a technique that became fashionable in TV documentary interviews of this period, that of shooting the subject from fifteen angles at once (including at least one that is badly framed and one that is distorted – we are only missing the out-of-focus and the wobbly, hand-held black-and-white shots here) and cutting relentlessly between them. For a film-maker of Morris's experience and skill this was obviously an artistic choice, but in lesser hands it always comes across as the mark of a director who either doesn't believe that their subject is interesting enough on their own, doesn't trust the MTV audience to sit still for a shot of more than five seconds in length, or is desperately trying to attract attention to themselves in a bid to make a move to directing music videos. Either way, it's very irritating and distracts from the information we are supposed to be absorbing, something The Thin Blue Line never does. Framed 1.85:1, the transfer is fine, but non-anamorphic.

There are no two ways about it, The Thin Blue Line is simply one of the finest documentaries ever to grace the big screen. Beautifully devised and edited, it makes for gripping viewing and did what all social or political documentary film-makers must aspire to – it made a difference, not just to Randall Adams, but to cases that followed in its wake that would not be so easily railroaded, and to the public's attitude to their own justice system. MGM's disk may be light on extras but it finally delivers the film in its correct aspect ratio and makes it more widely available for the modern, documentary-aware audience to appreciate and admire.