Most movies about time travel are bullshit. They can't make any sense because literally time travel cannot make any sense. That's ok. Terminator is hugely fun. but it's not really about time travel, that's just a function of the plot to get a cool robot chasing down the heroes.
Primer tries to make sense.
It's pared down, cheap, wordy science fiction. And absolutely fantastic.
Thrilling in ways that most action movies are not, it's baffling and complex whilst being clear in purpose. I may have needed a graph to fully grasp the intricate nature of plotting but it felt earned and not like it was cheating at every turn.
The performances are good, it's crisply filmed and incredibly clever.
And clocks in under 80mins. Superb.