Even within the realms of science fiction, time travel requires a big leap of faith to swallow. The potential problems of paradox are often acknowledged, but the fine detail is too often skirted around, leaving sizeable logic holes for the sceptical to pick at. The problem with time travel in general, of course, is that it is theoretical physics, an area of science whose concepts exist only as postulations unsupported by conclusive evidential data. It's the scientific equivalent of religious faith, requiring only belief to make it possible and therefore probably true, with the as-yet still speculative concept of the multiverse cited to cope with the problems that travelling backwards would pose to the continuity of the present day, and the notion of rigidly set destinies that the ability to hop forward would suggest.

It's rare for any film to explore the possible implications of time travel in any depth – for the most part it functions as a device to complicate impending relationships or to be tinkered with by the dastardly to threaten the future. So to discover a film that not only tackles the subject head on, but does so with real intelligence and almost mind-boggling complexity, is a rare and wonderful thing. The fact that it will probably leave you baffled on the first viewing should not be taken as a criticism of either the film or the viewer. You'll need to see it more than once. Like its lead characters, you will need to travel back to the start and re-examine events with an eye for what is to later occur, watching for clues that help piece together what at first will likely prove a mystery. If ever there was a film that was perfect for the multi-view ease of the home video format, then Primer is it.

The story revolves around a small group of young scientific engineers who spend their spare time experimenting with technology in the garage owned by one of their number, Aaron. He and his close friend Abe have a new project under way, the construction of a machine that affects the earth's gravitational pull on small objects. But the device has an unexpected side effect on an object placed inside it, which after only a few minutes emerges wearing two years' worth of fungal growth. Further tests confirm that the two men have accidentally created a field that allows objects to travel backwards in time and return to the present in a potentially never-ending loop. They theorise that if you were to exit the loop before it completes its journey, then you could emerge at a set point in the past. The next step, of course, is to build one large enough to take a person...

Now when I said that the film explored the issue of time travel in a complex manner I wasn't joking. Even the above set-up to the main story – which explores not just the potentially paradoxical issues of the time travel process but the nature of friendship, trust, and life control – lays nothing out on a plate. There is little in the way of traditional exposition and the dialogue is laced with technical talk that few will be able to follow in complete detail. But as with the scientific banter in Altered States, this doesn't matter a jot – although some key plot details are encased in this banter, its initial purpose is to create a sense of character and situational authenticity, to convince us that we are not listening to actors but to real scientific engineers talking real mechanics. Even the fungal growth is explained in biological terms, referred to here largely by its Latin name of aspergillus ticor. But believe me, this is the easy stuff. The film may only run for an unusually brief 77 minutes, but crammed into that is a good 3 hours' worth of plot, which has been been trimmed of its fat and any clear explanations, and it is left to the viewer to unravel the complexities of just what has transpired. In the later stages we are dealing with up to three versions of the two main characters at any one time, with little obvious indication of which is which and who has travelled back from when. or indeed, whether the reality we are witnessing is the same one in which the story and characters began.

When we screened the film for a film society cinema audience, I was aware of its reputation and was determined to stay with the plot throughout, no matter what. Well I was doing fine, at least I thought I was, until about fifteen minutes from the end. This is the point at which the film-makers effectively throw down the gauntlet in what almost feels like a direct challenge to the audience to work out just what the hell is going on. As the lights went up, I turned to those behind me and saw nothing but bemusement. I have yet to meet anyone who fully understood what they had seen on the first viewing. Some will argue, of course, that if an audience is left confused then it is because the director has failed to successfully communicate his ideas, but I have no doubt that this very argument is used by the Hollywood hierarchy as justification for the dumbing down of anything considered too smart for the slowest member of the blockbuster audience. If we are to say that a film should not be too complex, who is to set the bar that film-makers should not go beyond? The process of unravelling the plot is one of the very real pleasures of Primer, and one that stays with you long after the final credits have rolled.

But what really moves the film into a league of its own is that although second and third viewings do clarify some issues, they further complicate others, throwing up questions that had passed me by on earlier screenings and causing me to completely rethink what I was convinced I had understood the first time around. I was five viewings in before the everything started to really fall into place. Well, not quite everything.

The key thing is that even after that first screening I still, despite my confusion, felt electrified by what I'd witnessed. I may not have understood everything, but I absolutely knew that the answers were there and that given time I could solve the riddle. And I wanted to – I genuinely couldn't wait to see it again, to have another stab at putting the pieces together. Even now, a further four viewings later, there is just enough left unexplained to allow room for speculation and debate. When was the last time you shared beers with friends and excitedly discussed the possible readings of the plot points of a film you'd all seen at least twice?

Shot on Super-16mm for an almost microscopic $7,000 by engineer turned self-taught film-maker Shane Carruth (a genuine auteur in that he is writer, director, editor, co-cinematographer, lead actor and score composer), Primer is a brilliantly devised, low-tech genre film that dares to be challenging and intellectually stimulating without feeling the need to dress it up with cinematic eye candy. Now THIS is science fiction. And how.

As mentioned above, the film was shot on Super-16 and was blown up to 35mm via a digital intermediate, resulting in a slight loss of definition in places, something that is more pronounced on the cinema screen than on the DVD. In other respects, the transfer is very good, with the film's sometimes stylised colour scheme accurately reproduced and the detail actually superior to that on the New Line region 1 release. Some of the night-time exteriors are awash with what looks like grain, but Carruth informs us on his solo commentary track that this was the occured during the digital intermediate stage of the blow-up process, and that the Super-16mm original looks fine. The framing is 1.78:1 and anamorphically enhanced.

Primer breaks with Tartan's recent run of DTS tracks by only including Dolby 2.0 stereo, which is faithful to its film original (the film was edited and mixed on Carruth's home computer). Dialogue is not always as clear as it could be, the result, we are told, of first-time film-makers not investing in decent sound recording gear and having to re-record about half of the dialogue, usually in Carruth's apartment. The music and sound effects are very clear, though.

The extra features here appear to be largely identical to those on the American New Line disc. First up is a Director's Commentary with Shane Carruth. Now those of you hoping for an explanation of the plot complexities are in for a disappointment, as he largely steers clear of this in favour of detailing the technical aspects of the production, the help he received from friends and family, and the guerrilla nature of much of the exterior film-making. As someone who firmly believes that while low budget film-making may have its drawbacks, it's still way more fun to be involved in than bigger budgeted industry productions, I get a real kick out of stories of on-set improvisation and ingenuity, and this commentary is loaded with them. There's also some explanation of the science behind what the guys are up to in the first part of the film, adding to the sense that they are involved in real scientific engineering rather than a scriptwriter's technobabble. It does become clear just how tight the budget was here, with almost no traditional coverage and almost every frame of shot film used in the final edit (Carruth even points out where he says "Cut" in one shot). He's also amusingly critical of elements he is not completely satisfied with, twice saying of his own work "I don't know what I was thinking..." This is a very busy in fascinating commentary, with Carruth hardly pausing for breath in places, though there are a couple of small gaps later on.

There is also a Cast and Crew Commentary, featuring Shane Carruth together with co-cinematographer Anand Upadhyaya, location sound recordist Reggie Evans, production assistant David Sullivan, lead actor David Sullivan, Shane Curruth's father Chip Carruth (who plays Thomas Granger and supplied food to the production), and camera operator Daniel Bueche. Considerably less informative than Carruth's solo commentary, this nonetheless communicates well the team spirit of the production and the sense of fun I mentioned above. And it is infectious – at times this is almost like listening to close friends sharing lively stories about their summer vacation. Nevertheless, information on the production is intermittently forthcoming, even if some of it – the importance of the food constantly supplied by Shane's parents – is anecdotal rather than technical. Intriguingly, the director talks twice of the 'making-of' featurette he was at this point in the process of preparing, a supplementary feature that appears neither on this disc nor the New Line release.

A quick mention should go to the sound quality on this commentary – while the voices of some of the participants come across very clearly, others sound almost as if they have been run through a reverb filter, and one that is slowly turned up as the feature progresses. It is also worth noting that you cannot switch between the soundtrack and the commentaries while playing the film itself – this can only be done on the Extra Features menu.

Finally we have the Trailer (1:39), which is non-anamorphic and, curiously, framed 2.35:1 – presumably the marketing people believed that the audience expects science fiction to be in scope only. It's still an intriguing sell, oddly based on Maslow's Hierarchy of Needs.

There's also a Tartan Trailer Reel, the standard 'look what else we sell' extra.

With so few recent American films really requiring you to think and think hard, Primer is a real pleasure, a science fiction film that is as intellectually complex as the best genre literature. Delightfully low tech (even its one brief visual effect is bargain basement stuff), it somehow sells the time travel concept as plausibly as it does its accidental discovery. Please, someone, don't put Shane Carruth in charge of the next Batman movie – just give him some money and leave him alone to make his next film the way he wants to. The world will be a better place for it.

Tartan's DVD, like New Line's region 1, seems to be missing a featurette that Carruth was in the process of putting together when the commentaries were recorded, but otherwise serves the film well, with the picture looking as good as or perhaps a little better than it did in the cinema, and two commentaries that may not help to unravel the plot, but are still informative and fun. Highly recommended.