Primer is the most realistic science fiction movie I've ever seen, and hands down the best time travel movie of all time! Many time travel stories depend on invoking either nostalgia for long-passed days or dystopian fear of distant futures. Still others depend on far-fetched occurrences and/or special effects to achieve their goal. Primer, on the other hand, is in a class all its own. To begin with, the time travel destinations are only hours or days removed from the present. This is part of what gives the film its very puzzling quality, as people do not age noticeably nor do environments change drastically within a few days time span. As a result of this, the chronology of the film must be deduced primarily from the behavior, intentions, and dialogue of the characters; behavior which is unpredictable, intentions which evolve as the film moves along, and dialogue which is awesomely scientific. As other reviewers have pointed out, the writer/director/protagonist/composer makes no attempt to dumb down the conceptual framework and overall temporal complexity within which the film operates. Primer is difficult to understand. Whether you are a scientist or a science fiction fan, the first time you watch it you will probably only fully comprehend about half of it. But this, for me, is the film's best quality. The urge to rewatch it is immediate due to the fascination invoked by what was not comprehended. At this point I have watched it several times, and each time it makes more sense to me, however I still do not fully comprehend it.