There's a moment in Shane Carruth's impressive debut feature, Primer, that sums up the filmmaker's point-of-view quite nicely. It's not the line quoted in many reviews, where one addled engineer asks another, "Are you hungry? I haven't eaten since later this afternoon," although that line exemplifies Carruth's prickly wit. It takes place before the main characters' great, accidental discovery when two men discuss NASA's efforts to develop a pen that would work in zero gravity. After spending millions trying to work out the problem, the agency ended up using a simple pencil. Primer is the science-fiction film equivalent of that pencil, and the remarkable amount of bang the film gets for its buck is only one of its virtues. A less imaginative filmmaker with a big budget at his or her disposal would have started the film where Carruth ends Primer, a precarious moment when it seems the world is about to change. But Carruth is more interested in intimate moments of discovery, as the moral and philosophical implications of Abe (David Sullivan) and Aaron's (Carruth) invention reveal themselves. Because Carruth trusts his audience's intelligence, he allows us to share in these discoveries, rather than spelling everything out. Which is all well and good, to a point. But even the most attentive viewers may have trouble finding their bearings as the film opens with a quick flow of engineering jargon, and anyone who can follow the temporal twists and inversions of the film's final third probably deserves an engineering degree. Primer is a witty, well-shot, and convincingly acted science-fiction brain twister, leading one to suspect that on repeated viewings a rock solid narrative will reveal itself.