The action is set in the modern day, but this qualifies as "Science Fiction" because our protagonist (who is called "the Protagonist" and played by John David Washington) finds himself dealing with the un-firing of bullets ten minutes into the movie. We get a lecture about the physics of reversed entropy - how we can have bullets that go backwards through time. This is classic Christopher Nolan, and somewhat reminiscent of his first film - which essentially ran backwards through time. This is ... quite different. Much more grandiose, and more logically damaged.
I can't tell you more about the plot without getting into spoilers - I also can't tell you more about the plot and continue to make sense. Nolan uses the whole movie to get you adjusted to the ideas involved, so you can sort of vaguely understand the crazy ending (which of course can also be argued to be the beginning). But just like the equally grandiose and crazy "Inception," the people who love Nolan will love this, and I'm left scratching my head at the absurd, but so beautifully presented, dog's breakfast of logical problems he thinks makes a whole. I have to admit, I kind of enjoyed it - but I get pissy because he's built a house of cards that was grand to watch in the construction but has collapsed by the time he turns around to say "see how cool it is?!"