What happens if it actually works?
Why it is hard sci-fi
Two engineers in a garage discover a time-travel side effect of a superconducting box. The film is famous for refusing to translate its own jargon. The protocol for using the boxes runs through doubles, fail-deadly conditions, and recursive timeline edits, and the film never simplifies the description. Carruth shot it for seven thousand dollars and gave it the texture of an actual unauthorised research program.
Science inside it
Closed-loop temporal recursion using a continuously running box. The user enters the box at time T_end, and exits at the time the box was first switched on, which is T_start. Run the box for six hours, you can travel back six hours. Stack the boxes, put boxes inside boxes, and the timeline starts to fold. The film is rigorous about that grammar even when it is unwatchable on first pass.
Spoiler alert
By the third act the protagonists have run so many concurrent loops that they are no longer the original versions of themselves and cannot agree on what happened. One of them rents a warehouse abroad to build an industrial scale box. The film ends mid-sentence on purpose. The narrator s loop is no longer closed.