To those growing up in the 1950s, I am assured, Norwegian biologist Thor Heyerdahl was an almost mythic figure. An adventurer in the true and even heroic sense of the word, he was prepared to put his money where his mouth was and lay his life on the line to prove a theory. Allow me to explain. Having observed striking similarities between ancient sculptures found in the Polynesian South Sea Islands and relics of vanished civilisations found in South America, Heyerdahl began looking for a possible connection. Following detailed research, he came to believe that the islands could have been settled by ancient civilisations from South America long before the arrival of Columbus, traveling on simple craft using the easterly trade winds and sea currents. He published his theories as a thesis, Polynesia and America – A Study of Prehistoric Relations, but was soundly scorned by the scientific establishment, who stated categorically that such a racial migration at that time was inconceivable. The only boats recorded to have existed on the coast were primitive, sail-driven balsa wood rafts, vessels that would, it was believed, be difficult to control and would collapse as soon as they hit open water, and would certainly not survive the 4,300 nautical mile journey to the Islands. Heyerdahl thus decided to put his theories to a practical test by building his own balsa raft – one based on descriptions recorded by the Spanish on their arrival in the Americas and employing only materials that would have been available at the time (no nails or screws were used) – and together with five colleagues attempt to recreate this theoretical journey. He named the raft after the ancient Inca sun god, Kon-Tiki.

Now you have to get this in perspective to realise the scale of such an undertaking. This was back in 1947, when Heyerdahl and his crew had no satellite navigation and only a low power HAM radio for communication (one powered by a hand-wound generator), and no fleet of support vessels hovering a few miles back in case they ran into difficulties. When he and his crew disappeared over the horizon line, they were on their own, their position unknown, their fate very much in their own hands and that of their hand-built craft. Though the expedition attracted international interest, the expert opinion was that it was doomed to fail, that the boat would break up before even half of the journey was complete and that all lives would probably be lost. One company even promised the whole crew a lifetime's supply of whisky if they survived. Boy must their faces have been red. For the crew, the stability of their craft was only one of the hardships they faced – shadowed by sharks and whales and battered by storms, they had to eat what they could catch and drink diluted sea water and even fluid from the lymph gland of a fish in order to extend their fresh water supply. After 101 days at sea the crew finally sighted land, and in an ironic final twist, became shipwrecked on the rocks of the uninhabited Polynesian island of Raroia.

Heyerdahl's literary record of the expidition, titled Kon-Tiki, was translated into sixty-seven languages and sold in excess of twenty million copies. In 1951 this film, assembled from footage shot by Heyerdahl and his crew before and during the journey, was released to widespread acclaim and won the Academy Award for Best Documentary. It is this last fact that will inevitably prompt specific expectations from modern viewers coming at the film on the back of the recent spate of technically eye-catching documentary features, from Bowling for Columbine to Super Size Me and The Fog of War, expectations that will likely be disappointed. Kon-Tiki (or at least this version) opens with a written introduction from Heyerdahl pointing out that the footage was shot by the crew, all amateurs in the field, but that "what is shown, however, is what actually took place." Authenticity is from the outset identified as being far more important than technical polish – all of the footage of the expedition was shot hand-held on a wind-up Bolex camera (the IMDB lists the negative size as 35mm, but 16mm seems far more likely) with no synchronised sound – Heyerdahl's narration was recorded in English for an international audience. The initial introductions are very formal, the narration delivered in what used to be called BBC English, and the explanatory map animations are fairly basic. The footage itself is often rough and ready and comes across almost as an adventurous home movie, complete with grainy, unsteady visuals and jump-cut editing. By modern standards, this is technically primitive stuff. So why was it such a success and why did it win the Academy Award?

It's actually pretty simple: the story. Kon Tiki worked for the 1950s audience and still works today because the tale it tells is so damned interesting. If the presentation gives the film the feel of a rather stiff 1950s BBC special, the sheer daring of the adventure eventually renders this irrelevant. Even the voice-over ultimately works for the film, making even the most extraordinary moments seem as everyday as they no doubt became to the crew, lending the film a nice level of understatement. If the home movie approach means that the epic nature of the trip is not really conveyed and the lack of sync sound prevents us really connecting with the characters on personal level, the film still enthralls in its incident and detail, from the flying fish that land on deck each morning to the sometimes alarming encounters with very large sharks, scenes that will strike a particular chord with those who were left rattled by Open Water.

There is the sense that the Academy Award was more a recognition of Heyerdahl's achievement than his self-confessed amateurism as a film-maker. In a post-modernist age when the technique of a documentary is celebrated as much as the content, the straightforward, even basic presentation of Kon-Tiki may well prove a barrier to some viewers, but it shouldn't. Whether it really is a great documentary work is a matter of opinion, but it is certainly an important and ultimately fascinating record of an extraordinary adventure, and is presented not as a series of retrospective interviews and re-enactments, but as a 1947 film equivalent of a video diary. As Heyerdahl reminds us, what we see on screen "actually took place," and that's as good a definition of film documentary as you could ask for.

OK, this was shot largely hand-held on a wind-up camera on the high seas, probably on 16mm, and is fifty-four years old, so it's never going to look pristine, but even allowing for that the presentation on Image Entertainment's disk is seriously sub-Criterion. There is a fair amount of dirt and some scratches, the contrast is variable (though largely OK) and sharpness is, well, a little lacking at best and poor at worst. Some of this is clearly down to the condition of the original print and as such is perfectly tolerable, but de-interlaced frame grabs display the sort of ghosting that suggests this NTSC print has been converted from a PAL video master, adding to the motion blur and fuzziness. It's certainly still very watchable, as you automatically make allowances for the filming conditions, but it could have been a lot better.

Sound is Dolby Digital mono 1.0, and is reproduced with very reasonably clarity, though is lacking in any real bass, not surprising given the age. As the soundtrack is predominantly made up of music, narration and ambient sounds, this is a serviceable transfer, though I suspect that the music and narration have been re-recorded and the soundtrack itself remixed for this version (see the following paragraph).

The running time here is 58 minutes, which would tie in with the original Norwegian running time (well, sort of, as that was 55 minutes), but not the version originally released in the US, which ran for 68 minutes. This version is identified as a television version produced by the Norwegian Broadcasting Corporation in 1995, who have replaced the original opening and closing credits with their own, and possibly rejigged the soundtrack. This would again hint at a Scandinavian PAL video source as the origin for this print.

Not a thing. There is not even a main menu, just a chapter selection. Given the wealth of material that could have been included, this must surely be a prime candidate for special edition treatment some day in the future.

An important and involving document of an extraordinary adventure, Kon-Tiki is a film somewhat lost in time, virtually unknown to a modern audience, who would have little chance to encounter it by accident, and dated enough in style for few to actively hunt it out, especially given this disc's lack of features and less-than-perfect transfer. If a UK release should ever turn up then I'd urge those interested in the documentary medium to at least give it a look, as its low key presentation is actually a welcome change from the forced emotion and thunderous dramatics of many more recent documentary works. In the end it stands as a cinematic testament to the boldness and determination of Heyerdahl and his crew, and the lengths this extraordinary group were prepared to go to just to prove a theory.