You probably have seen other time-travel movies before. If you have not, you should. Anyway, this is another time-travel movie. Just to avoid any confusion, time-travel in Primer is a single timeline where persons go physically and mentally back in time with the same memories and knowledge and exist simultaneously as the future self.
Well, it is a cold movie indeed. After all most of the movie is about two engineers, colleagues and friends accidentally inventing a time-travel machine and the privileges, problems and dilemmas the machine brings with it.
Everything begin with a guy talking over the telephone narrating a story continuing through all the duration of the movie. There is a team of four colleagues working on some profitable side-projects aside from their normal jobs. Two of them start working on a little more secret project where it accidentally became a time-machine. Their reactions are calculated and totally human and scientific. The questions asked, the real-life planning and the reasons behind every action were true, logical and humane.
Of course at some point as most of us would do, these two friends want to make money. A lot of it. And they succeed. Bit by bit, in order not to attract attention, they take advantage of the time machine by acquiring knowledge to grab money making opportunities like buying and selling stocks and placing bets.
Soon problems arise and the character of each one of them separately starts to matter. A little spoiler here but the movie has not a very good end. Not an acceptable good end at least. I can not speak very subjectively but it might be useful to watch the third act a little slower, re-watch it, or be very careful of what is happening because the plot starts getting tangled very fast and many hypothesises are placed during the phone call and the planning.
In the end the reason you can watch Primer is the authentic and logical flow of events that take place. These two protagonists act like everyone of us the viewers probably would. It is the closest and most realistic approach if time-travel existed and if a small team of people invented a time-machine in the beginning.
"Aaron: What's worse: thinking you're paranoid or knowing you should be?"
"Abe: Aaron, I can imagine no way in which this thing could be considered anywhere remotely close to safe. All I know is I spent six hours in there and I'm still alive... You still want to do it?"