Time runs out.
Why it is hard sci-fi
Nolans grammar of inverted entropy is the films working physics. Objects and people can be inverted by passing through a turnstile, after which they move backwards through time at normal subjective speed for them, but in reverse for everyone else. The film commits to the rule and stages a final battle with two assault forces, one running forward and the other running backward, on the same field at the same time. The rule is rigorous and the staging is careful.
Science inside it
Local entropy reversal as a fictional physics, the implications of inverted travel for breathing, friction, and combat, and a temporal pincer movement where information flows in both directions at once. The film cites the grandfather paradox and chooses a working answer. The branch and the trunk co-exist.
Spoiler alert
The Algorithm is a future device that, when buried at nine specific sites, runs entropic inversion on the entire planet. It is built by future humans who blame the present for ecological collapse. The films closing reveals that the protagonist is the future founder of the operation that recruited him, which is a closed loop the film commits to without flinching.