Shot for a mere $7,000, Primer is about time travel. Sort of. It's more about the breakdown between two people, but an incredibly confusing causally linked time travel mechanism underpins it. If someone tells you they understood it on their first viewing, they're filthy liars. Written by a mathematician/engineer, none of the jargon or lingo is cut, making it as factually accurate as one could imagine a time travel story to be. The plot loops in on itself in recursive and terrifying ways. Trying to follow it? Here's a sample timeline, here's another, though this one is the most accurate. Yes, it really is that batshit confusing, but watching it over and over to pick it apart is surprisingly fun. Unlike some of the other films on this list which are confusing just to be confusing, Primer actually makes complete sense, if you're willing to put enough time and effort into it to understand what's going on.